A free bet can come in many different forms and can be claimed by both new and existing customers, but generally speaking, if you hear an operator or affiliate referring to a free bet, they are normally talking about the welcome bonuses being away to new customers.
Free bets typically come in a “bet and get” format, so you deposit and bet £10 for example, and you receive £30 in free bets. These free bets would then be subject to certain terms and conditions, which vary depending on the operator.