Mystery bonuses can be triggered by randomly selected times, amounts of play or jackpot size. On games with skill-based bonuses, skilled players will do better than unskilled players, but do not have an edge on the house.
Bonuses usually come with wagering requirements, which means that you have to bet a certain amount of money before you can withdraw any winnings that you make with the bonus money. Wagering requirements can vary widely from one casino to another, so it's always important to do some research.