The act of giving a man oral sex. This act should ALWAYS be reciprocated by going down on the giver of the initial blowjob.
Why? Cause it tastes like shit, it's not fun, very few girls enjoy giving them, and we deserve to be repaid.
Keep a girl happy and she'll be willing to do things like this for you. The happier she is, the more she's probably willing to do.
Guy: "Holy shit that was an awesome blowjob! Let me get to the point where I can move again, and it'll be your turn."
Girl: "Sounds good to me!"