An action usually required for a bonus.
Rollover is basically a wagering requirement where you have to place bets with a certain stake and odds in order to play it through and get your money. See also: qualifying bet.
Let's say that a bookmaker gave us a $100 bonus with a required 3x rollover. What does it mean? It means that bonus money will be «cashoutable» after you will place bets worth of $300. It is up to you how you are going to do it, you can either place two $150 bets or fifteen $20 ones.