FreeBets are available only on sport betting to place single and multi-bets, but not system-bets. There aren't wagering requirements, but you do need to use the whole FreeBet at once. If you have one to use, you'll see it in the upper right-hand corner of the sport betting pages.
A FreeBet is an amount that can be used to place a sports bet. If your bet wins, the net winnings (winnings minus stake) will be credited as cash into your account balance.