Provably fair describes games whose results players can check for themselves using cryptography. Before a round, the casino commits to a secret server seed by publishing its hash. The player contributes a client seed, and a counter called a nonce changes with every bet. The outcome is calculated from these values, and once the server seed is revealed, anyone can rerun the calculation and confirm that the result was not altered after the bet was placed.
The model grew up alongside crypto gambling and is common in crash, dice, plinko and mines games. For operators, it is a strong transparency feature for a crypto casino, but it does not replace regulation: licensed markets still expect certified RNGs, independent game testing and approval under their own rules.