In blockchain technology, a hard fork (or hardfork) is a significant modification to a network’s protocol that makes previously invalid blocks and transactions valid, or vice versa. A hard fork necessitates that all nodes or users update to the most recent version of the protocol software.
Forks may be initiated by developers or members of the cryptocurrency community who are dissatisfied with the features provided by current blockchain implementations. They may also develop as a means to crowdfund new technological ventures or cryptocurrency offers.
A hard fork comes into play when nodes of the most recent version of a blockchain no longer accept the prior version(s), resulting in a permanent divergence from the previous version of the network.
Adding a new rule to the code effectively causes a split in the blockchain, with one route following the new, updated blockchain and the other path continuing along the old path. Generally, people on the old chain will realise that their version of the blockchain is obsolete or useless within a short period of time and will rapidly switch to the current version.