This type of restriction is the most common one you'll find with all types of bonuses offered by online casinos. Not only with no deposit bonuses. It's also known as playthrough or betting requirements.
The wagering requirements are the terms you have to meet to withdraw winnings generated by bonuses given to you. All top casinos, no deposit bonus offers, will have a playthrough requirement, and it is a multiplier attached to the number of times you have to bet the value of the bonus.
If a player doesn't meet this requirement, they will forfeit all other winnings during these bonus rounds. To give an example: if you have received a no deposit bonus worth 100 Pesos to play on Gonzo's Quest. To meet the minimum wagering requirement of 50x, you'll have to place bets to the 5,000 Peso value.
Once you've fulfilled the playthrough requirements, you'll be allowed to withdraw the winnings you accumulated.