A no deposit bonus is a bonus that you get without having to make a deposit first.
It is common for online casinos to give new sign-ups a no deposit bonus as a welcome gift before any 1st deposit has been made.
In some casinos, no deposit bonus money is given out to existing members as well to keep them happy and make them more likely to keep playing in the casino.
Always check the terms and conditions before accepting a no deposit casino bonus. Pay special attention to the rules pertaining to the withdrawal of any winnings derived from your no deposit bonus.
A popular alternative to the no deposit bonus is no deposit free spins, i.e. free spins that you get without having to do a deposit. Just as with a no deposit bonus, it is important to check the rules before accepting any free spins.