En Prison is a French term that refers to a particular casino bonus rule. The term is used in roulette and refers to the rule that allows players to have a second chance to win after placing an even-odd bet. Simply put, you can consider it a free re-spin.
Keep in mind that this isn’t a written roulette rule but only refers to the bonus offered by some casino establishments. En Prison is triggered once you place an even-money bet and lose to a 0 if you are playing European roulette or both 0 and 00 if you are playing American roulette.
If you trigger the En Prison rule, you won’t be required to add any more chips and your wager will remain on the table. However, one thing to know is that, while you do receive a free re-spin, you aren’t allowed to change the stake or the type of your bet.