I'll be honest, I don't personally understand Provably Fair more than at a fairly superficial level (I am not a developer). I know there are issues with our current implementation and they are already fixed on our latest software upgrade but that is presently still just on our testnet server and has not gone onto our live server yet but I assure you this problem should be resolved sooner than later. I apologize for the inconvenience in the mean time.
Just like nc50lc said. It's pretty simple.
Here you can find explanation (written in a way that average user can get it) together with example.
And few more links from BTT:
https://bitcointalk.org/index.php?topic=5184988.0https://bitcointalk.org/index.php?topic=161236.0