This is the Surprise Cashback, in which portions of you wager keeps accumulating, and will certainly come back to you at some point if you continue to play the games. It is a surprise so you don't know when your cashback falls out!
There's no wagering requirement for the cashback you will receive. You can use it as cash for playing live casino or withdraw them if you want! It is also a good deal that $5 will be already in your cashback balance upon your first deposit.
If you keep playing on games...
Some of the money you played with will go to Surprise Cashback
If you continue to play, the cashback that has built up will be issued in a big way some day!