A no deposit bonus is an incentive given by casinos to attract new players. By giving players a free trial, casinos hope they go on to make a real money deposit.
After you create a new account with an online casino, you will be given free spins or bonus credits that let you play a selection of eligible games. It is a common practice used by the majority of online casinos.
While you CAN win real money with no deposit bonuses, you do have to satisfy the T&Cs before you withdraw your winnings.