i have seen the phrase and it seems like it allows you to customize your army in some sort of manner, but i still don't get it

Doctrines are how IG are customised. See the IG codex for details.

Yeah, they basically give your armies certain abilities that others don't have. However, they usually limit your troops in one way or another as well. So, sometimes it's best, I think, to just pick a basic army and stick by it. Unless you really want an army of your own, in which case go for it.