This is the number of times you'll need to bet the bonus amount before you can withdraw it. For example, if a site offers a 100% match bonus up to $100, and the bonus wagering requirements are 5x, that means you need to wager $500 in total before you can withdraw any of the original $100 first deposit bonus.
You must wager bonus bets to turn them into cash. Then you'll keep any profit from those bets, but not the bonus bet itself. So if you put a $50 bonus bet on the Bucs to beat the 49ers at +400, you'd earn $200 in cash if the Bucs win ($50 x 4.0). If the Bucs lose, the bonus bet just goes away.