In multi-player games, having other people assist you in getting leaderboard position or achievements (xbox 360 specific) by having other people help you in ways that go counter to the game's design. Most often done with one team throwing the game to another.
Example: In Battlefield:Modern Combat, one Achievement acquired by hitting 5 enemies and 0 friendlies in one mortar strike. This is doable but, during normal play, exceptionally hard. Boosting would be getting 5 people you know to join the other team and all stand in one place so you could drop a mortar strike and get the achievement.
Getting to the top of this leaderboard is a joke. The only people on there did it by boosting.