Can someone explain support to me?

I'll be coming up on my first full year of playing 40k in April, and overall, as a newbie I did pretty good. But there's something I still don't understand. When reading tactica or tactics, I'll see it said, "Support such and such unit" but they never really define what that means.

Now I understand the basics of support.

I.E. If I attach a haemonculus to a unit I'm supporting them with a pain token, if I attach an archon with PGL to a squad of incubi that's supporting them, and a Cronos Parasite Engine is nothing really but a support unit.

What I don't understand is when it isn't so clear, and I think that may be why some of my units perform poorly or merely average. And this is a problem as I want to start running Harlequins, which I assume would need support, but how exactly is it provided to a standalone unit.

My current dilemma is say, with Reaver Jetbikes, they haven't really made their points back once. I hide them in cover, I put them out of line of sight, I surround them with transports to try to prevent templates from killing them first turn. They're faster than every unit in the codex, so how does one exactly support them with other units, unless you play so conservatively with them they still achieve relatively little?

How do you support a squad if you can't join anything to them, or give them a buff? And I know you can join units to anything not a vehicle or MC, but attaching an archon or haemonculus to a squad of reaver jetbikes or harlequins would hurt the unit overall if it remained attached.

I don't claim to be a great general, but support may not be direct by adding a character. I view support as not leaving a unit alone. A squad of Incubi is pretty awesome, but if left alone to hold a flank by themselves, they will probably fail. Also, a shooting unit with no CC ability could be susceptible to assault. Having a Talos or other scary CC unit nearby could keep the enemy from assaulting your unit.

Particularly with Reavers, if they are alone they are likely to die. However, if you have them near a Wraithknight the enemy will probably choose to deal with the wraithknight. If not, they will at least need to consider which target is more important.

^

Using Raiders full of Wracks to support your Haemy/Archon/Grotesque missile. All the other myriad options and combinations out there.

Basically, from the way I understand it, supporting a unit means not leaving it alone to die in a corner.
...Except for Mandrakes, who WANT to die alone in a corner.
Keep other units nearby that complement their abilities, or having 3 haywych venoms so if one blows up you'll still have more etc.

Basically the whole "No man is an island unto himself" concept.

I think "support" is a grey term, as it depends so much upon the individual units, and their applications, but yes, the idea is that you use things like distraction, shielding, tarpitting or softening-up to make two units better than the sum of their individual talents.

Force concentration is a very powerful tactic, for example:
2 identical armies are played against each other, Dave sets up spread along his table edge, then Sarah attacks at one flank; she pours twice the power into her attacks as he can in defence, thus she works her way across the enemy as he trips over himself trying to re-deploy.
As Sarah mulches each unit before it can retaliate, she rapidly gains table superiority. Dave loses too many tools too fast to be able to fight back, and loses in a landslide.

Support is about force concentration between 2 or 3 units: a squad of heavy weapons in the middle of the table is very offensive, but usually fragile; park a good combat unit nearby, and now the risks of assaulting the heavies is much higher.

Reaver support is going to be quite abstract, as you are correct: nothing keeps up with them for localised support. The key to supporting them is distraction; if they are in cover, far away, sneaking round to a deep objective, and there is a truckload of Grots running up the middle, the Grots are effectively supporting the Reavers, allowing them to move around unthreatened.

Personal opinion
Support, like most tactical play, is all but dead in this ruined game. Unless your group ignores all the recent power creep, and utter emphasis on shooting, concepts like support, doctrine and strategy have all gone out the window.
As the whole game has turned into "whose pew-pew rolls more hits", support is irrelevant. A fast combat unit like Wyches could dart ahead, tie-up a good unit, allowing a Talos to lumber up and finish the job; this was an excellent example of mutual support.
Now the Broadsides take out the Talos, the smart missiles and Ion charged templates vaporise the girls, and the game descends into a shooting roll-off.
Stupid GW have broken my game. Sulk. I'm off to play with my new Dreadball set!

Oof, someone's got serious tau beef. I'm lucky enough to have not played against them yet.

My pennysworth on support is much the same as the ones above, but remember to think about it in terms of the game you're playing.
"Not leaving a unit alone to die" is really the core of it, but support can also mean not expecting one unit to be able to leg it off the board with, eg. The Relic. When units support each other, what they're really doing is backing up your master plan in case (when) something goes wrong with it.

One example is an Archon with a webway portal - he indirectly supports every one of your Reserved units by giving them an alternative place to pop out of should your own board edge become mega dangerous.
I've seen Mushkilla give a nefarious example of direct support by placing a Venom in between the squad that got out of it and an enemy squad who wanted to assault them. The enemy couldn't charge through the venom,, and that squad could move about as it wanted, safe on at least one side.
Either of those things could work with Harlies, as could a million others. I think because they're pretty fragile, a transport and a distraction are very necessary.

For better examples of why reavers don't need support (except from each other) than I could ever give you, look at Mushkilla's battle reports with the Black Buzzards on this forum. I've learned so much. Bladevaning runs that end in safe cover are key to everything.

Well, it's good to know I've been doing some of these things for the most part. I suppose I meant more in terms of list building.

If I'm running harlies, I'd probably screen them with a raider if they're not in area terrain to give them a 2+ cover save, but I'm not adding a raider to the list for that purpose it just sort of works out that way.

Perhaps their failures (the reavers) are due to the fact my list is so mechanized they're the only vulnerable infantry until my boats are destroyed or something has disembarked. But even in that situation I deploy them out of sight and try to keep them out of enemy range. I generally do end their bladevane in cover, but I must ask, what's the point? It doesn't increase their cover save to a 2+ or anything, it just helps keep them from being assaulted, which, you shouldn't park close enough to be assaulted anyway.

I'm quite adept at supporting units with what I have on the table... I guess. But how do you look at your list of reavers and say, "Okay, I need Grots to make this work."

You're right, I didn't mean "cover" in the rules sense, I meant out of LOS.
Damn words. So tricksy.

Support during list building is a function of your plan. While making a list you should have a plan for how you believe any given game will work; usually at least one plan per mission type - modified based on the make up of the armies you think you will face.

For example, my plan for the objective based missions is to make sure I have plenty of scoring units. So my list starts with 4 or 5 units of warriors. Those warriors will need 2 forms of support. The first is a way to get to the objective, so the will have transports. The second is to provide targets that my opponents might end up prioritizing above them. For example a talos/cronos or reavers.

Deploying two cronos in the middle of the table represents two hard to kill heavy hitting targets. If my opponent is shooting at them then he's not shooting at my troops. I don't care if those units "earn" their points back as by simply absorbing fire they are acting as a shield to my more fragile units which is way more important.

Now if I expect to see knights or lots of mech then I'll trade out some warriors to be haywyches instead. They are still scoring but can now play a different role.

Of course I have to temper some of the above selections based on game size and other needs. For example if I'm taking reavers anyway I might put blasters on some of them so they can be tank killers instead of using wyches.

Point is: your support units are determined when you are list building based on what your overall plan with the army is.