The Sign-up bonus is a casino bonus you get as a reward for signing up with an online casino. It is exclusive for new players and can only be claimed once. The sign-up bonus is often free spins on specific slot machine games that the casino specially want to promote.
Just like casino bonuses in general there are terms and conditions that you need to pay attention to. Wagering requirements on the winnings you eventually make are common and also winning caps are used to limit the risk for the casinos. The Sign-up bonus is mostly to be seen as a great opportunity to try out new casinos.