What is Nonce?

Because the nonce is only 32 bits, roughly four billion values, modern miners exhaust it in a fraction of a second. They then change other parts of the candidate block — the timestamp, or the coinbase transaction, which alters the merkle root — and search again.

The nonce is the clearest illustration of what mining is: not computation toward an answer, but repeated re-rolling until a random output lands below a threshold.