Sportsbook give away bonuses in order to attract players to their sportsbook.  Usually this bonus is a % of the total deposit. In order to keep people from depositing a large amount and maybe placing a bet or two then withdrawing the money (including the bonus money) it has become industry standard practice to institute a “rollover”.  For a better explanation here is a quote from a popular source:

— For a list of approved sportsbooks click here —

How it Works…

Roll-Over basically means you must risk the amount (deposit and bonus) multiple times to benefit. For example: In the Sportsbook, if there is a three (3) time roll-over requirement…. if you deposit $100 and gets a $25 Free Bet bonus, you need to wager a total of $375 before you can withdraw.

Sportsbook

1. 1. Roll-Over is defined as the amount of cash action risked or won, depending on which is the lower amount on qualifying bet types and lines.
      For example: If you place two bets,
      Wager A: $110 to win $100
      Wager B: $100 to win $140
      The win amount from Wager A would be added to the risk amount from Wager B to give you a total of $200 towards your roll-over requirements.
   2. Wagers that are canceled, a push, no action or pending do not count towards roll-over.