It's a bonus you can claim without having to deposit any of your own money. A no deposit bonus is usually significantly lower than a deposit match bonus (where you're required to deposit a certain amount to claim the reward on offer). But if you're new to online casinos, or you just want to dip your toe in without risking any of your own cash, a no deposit bonus is a great way to get a feel for the real money gaming action on offer. Plus you could end up winning some real cash in the process too.