In online games and tournaments each player has a set amount of time. When you make your move, your clock stops and your opponent's starts.
Time controls are written with two numbers: 3+2 means the game starts with 3 minutes and 2 seconds are added after each move. 10+0 has no added time.
If your time runs out, you lose. One exception: if your opponent has no pieces left that could mate you, the game is a draw even if your time runs out.
Bullet is under three minutes, blitz is three to eight minutes, and rapid is longer. If you're new, 10-minute games are best for thinking.