A rollover is a multiple of a deposit that you must bet in order to cash out any winnings from that sportsbook. If you deposit $200 with a 10x rollover bonus, you need to make $2,000 worth of bets to ever see that money. It sounds harmless since you likely deposited at that sportsbook to bet, not win once and run.
Yes you can make money laying horses on Betfair but it is not as easy as you think many punters wander into this with the understanding you have typically N-1 runners in horse race on your side where N is the number of horses in the race and 1 is the horse you have layed to lose the race.