No-deposit bonus
A small bonus or a few free spins you get without depositing, almost always with tough rules.
A no-deposit bonus gets you a bit of bonus cash or some free spins just for signing up, confirming your email or typing in a code. You don't put in a cent.
Free, but the terms are brutal
It's a no-risk way to check out a casino. But the rules are the strictest of any bonus: high wagering, a short deadline, a low max cashout and a bunch of games you can't play. Lots of sites also want you to deposit before your first withdrawal, even when it's no-deposit winnings, and most will ask for KYC.
Real-life example
You sign up on your lunch break and get 20 free spins at 0.20 $. You win 8 $. That 8 $ becomes bonus money with 50x wagering (400 $ in bets) and a 50 $ cap. Even if you clear it, you still need to deposit 20 $ before you can withdraw.
What it's good for
Testing the site's layout, games and support. Not for judging withdrawals: the extra rules tell you almost nothing about how a normal cashout gets handled.