A block header is section in a block containing metadata and a summary of block’s transactions. This is hashed repeatedly to create proof-of-work.
It is a means for identifying individual blocks generated inside a blockchain network, with each block containing its own personalized header to facilitate the tracking of protocol modifications. It is used to handle all blocks in a blockchain, which are also known as nodes. Each block header contains all of the metadata, including the time and difficulty of the block’s mining, the Merkle root of the contained transactions, and the nonce.
A block header is hashed periodically by miners by changing the nonce value as part of normal mining activity. Their goal is to build a proof-of-work through this exercise.
Block headers, in summary, are used to target certain blocks inside a blockchain. They contain useful information about each block and miners hashed them to provide proof-of-work.