If you have never heard the term, boosting is a way of getting together with other gamers to get achievements without actually having to do the hard work of earning them. For example, one achievement in Halo 3 is to get a triple kill (kill three opponents within four seconds of one another) in a ranked free for all game. You could do this by playing a game and trying to lure people together and then attacking them, or firing a rocket into a crowd of people, or a variety of other means. It may take a while but eventually you will get there. Alternatively, you could ask three people to stand in front of you and shoot them at point blank range.