The word nonce means a number used once. In provably fair systems it is a simple counter. Your first bet with a seed pair is nonce 1, the next is nonce 2, and so on. The site hashes the server seed, the client seed and the nonce together to produce each result. Without the nonce, every bet on the same seed pair would give the same outcome.
Why does a provably fair system need a nonce?
Seeds are set once and used for many bets. A server seed is committed by its hash before play, and a client seed is set by the player. If those two values were the only input, the formula would return the same number every time. The nonce adds the round number, so bet 7 and bet 8 come out different even though the seeds have not changed.
The nonce also makes verification precise. When you want to check a specific round, you look up its nonce, plug in the revealed server seed and your client seed, and rerun the formula. The result should match what the site showed for that round. If the site's nonce counter skips a number or repeats one, that is a sign something is wrong.
What does the nonce tell you about a session?
It tells you how many bets you placed on the current seed pair, which is useful in itself. People often lose track of how many rounds they have played. A nonce in the hundreds after what felt like a short session is a plain fact worth noticing.
Like the seeds, the nonce has no effect on odds. It only makes each roll distinct and checkable. The chance of a good result and the size of the payout are set by the game's rules, and the house edge built into those rules means the expected outcome over many nonces is a loss. Use the provably fair verifier to replay rounds, and see responsible gaming for tools that cap sessions.