A no-deposit bonus is free money (real money) that some online casinos give you to try their games. The casino usually has some requirements for you to play a certain amount before you can withdraw the money. Ensure that you read the requirements.
Sound too good to be true, well it isn't. Receive a free bonus and play with the casino's cash. No need to send them any of your own money or give your credit card.
You can cash in these bonuses and receive money from them. If you win and follow the promotion terms (minimum play) they will send you cash. The casinos provide these bonuses to encourage you to try their games.