How does Poison Work?

Hey everyone. So I am new to 40k and chose the Harlequins and Dark Eldar as my jumping in point. I have been reading a ton on reddit, this forum, and my index and I keep seeing "poison" referenced a lot in regards to shots by Kabalite Warriors, Venoms, etc. I can not for the life of me find anything that says what poison does or how to even put it on my units gear. What am I missing?

Poison wound on a fixed number (mostly 4+) on non-vehicule target and 6+ against vehicule.

Ah okay. I see where that applies now. Thanks for clearing it up for me. I figured it would be something super simple I was missing.

In the previous edition Poison was a universal special rule, which meant most of our weapons would have something like Poison (4+) in the profile. This edition they have done away with universal special rules, and instead write out the abilities in full each time.

Basically, most commentators just keep the old terminology because almost everyone knows what it means. It takes someone new coming along to remind us that isn't necessarily the case.