A block header is the summary section of a blockchain block that stores metadata instead of raw transactions. It includes the block’s timestamp, mining difficulty, nonce, Merkle root of all transactions, and the previous block’s hash the element that links blocks together in a chain.

For miners, the block header is the part they repeatedly hash to validate a block, since it is far more efficient than hashing an entire block of potentially thousands of transactions. For example, Bitcoin block headers have a fixed size of only 80 bytes, making the process manageable.

Block headers are also useful for light clients, such as mobile wallets, which cannot store the entire blockchain. Instead, they only download block headers, which require minimal storage while still allowing users to verify transactions through Merkle proofs.

This system strikes a balance between efficiency and trust: while light clients depend on third parties for some information, they still perform basic verification, ensuring more security than relying blindly on others.