Comp is short for "Competitive", a game mode in
Counter-Strike: Global Offensive. Two teams of five play as terrorists or counter-terrorists and compete in a best of 30 rounds where the terrorists attempt to plant a bomb while the counter-terrorists attempt to repel the terrorists away from various bomb sites.
Players are ranked based on an elo system, from Silver I to
Global Elite. It is every
counter-strike player's worst nightmare to be placed in Silver I.