How Does a Change in a Block’s Nonce Affect Its Header Hash?

A change in a block's nonce drastically affects its header hash. The nonce (Number Once) is a random or pseudo-random number included in the block header.

In Proof-of-Work, miners iteratively change the nonce and re-hash the entire block header until they find a hash that meets the network's difficulty target (e.g. starts with a certain number of zeros). Even a single digit change in the nonce will result in a completely different, unpredictable block hash due to the properties of cryptographic hashing.