rollover usually refers to how many times you need to wager the amount deposited with a book in order to keep your bonus. example:
you deposit $1,000 and get a 10% bonus...which means you have $1,100 in your account day one. the book says you need to rollover this balance 3 times before withdrawing. depending on the book, you may have to make $3,000 in wagers or $3,300. also, the amount of the rollover may refer to the amount you have to "lay" or the amount you stand to win. some also require you to keep the money in the account and actively wager for 30 days (this rule sucks as it restricts you meeting the rollover in a week and making a withdrawl AND it also leaves it up to the book to determine what "actively wagering" means).
every book is different. if you are going for a bonus, make sure you completely understand the rules before you deposit or you could get screwed.
i would recommend staying away from a book that offers a bonus higher than 20%.