Rollover is the wagering requirement imposed by an online sportsbook that you need to bet before a bonus (or free bet) becomes eligible.
Rollovers are often bonus offers for making your first deposit at a sportsbook with the stipulation being that you have to bet a certain amount to receive the bonus.
For example, one site may have a $100 cash bonus and 5x rollover. That means if you deposit $100, you can get a $100 bonus, but per the terms and conditions, you won't be able to withdraw any money until you complete a 5x rollover from the deposit and bonus. That means once you get the $200 following a deposit ($100 deposit and $100 bonus), you'll have to bet $1,000 to be eligible for withdrawal, which equates from $200 x 5 = $1,000.
Every sportsbook has their own rules so this is far from the only type of rollover you'll see. There are also added rules like what kind of bets you can make and a set time period of when you can withdraw your money.
Rollovers tend to be more popular at offshore sportsbooks, while in-country operations like DraftKings and FanDuel use other types of free bet offers.