A free play bonus is a no deposit bonus that comes with a time limit. Basically, the casino will give you a large no deposit bonus, and 60 minutes to play that bonus on a select number of video slot games. After the 60 minutes, any amount over the initial amount of the free play bonus would be considered your winnings. If you start out with a free play bonus of $500, then your winnings would be any amount over the $500. Anything less than $500 and you would have no winnings. The winnings over the initial bonus are converted into a no deposit bonus.
The free play bonuses are popular at online casinos that are using the Microgaming casino platform. Some of the casinos offer free play bonuses as high as $3,000 but it's often difficult to leave a winner when you are only able to play a select number of video slot games with wagering limits and a 60 minute time period. The one advantage is that you get to try out the video slots for free over a 60 minute period.