A hard fork is a software update performed by the nodes of a blockchain.
Unlike soft forks, which provide backward-compatible updates, a hard fork will render the old protocol (which defines the rules) of the blockchain incompatible and the blockchain will cease to be used.
However, a lack of consensus within the community may result in the community splitting into two separate blockchains.
Each node keeps a copy of this digital register of data and contributes to the proper functioning and security of the network.