Block Hash
The block hash is the fingerprint of a block’s header and serves as the block’s unique identifier.
In depth
Each new block stores the hash of the previous block, which is what creates the chain. Since a hash changes completely if any input changes, altering an old block would break the link to every block after it. On explorers you will see block hashes starting with many zeros — evidence of the mining work that produced them.
Why it matters
The block hash is the glue of immutability: it is why history cannot be quietly rewritten.