The change in protocol of a crypto currency that results in the split of the blockchain, effectively creating a new blockchain so also a new currency.
The original currency still follows the old protocol, so no updates are needed by its users. However, the supporters of the new protocol have to update all nodes and also make changes to their mining devices etc ...
The most known Hard Forks are Bitcoing/Bitcoin Cash and Ethereum/Ethereum Classic.
Hard forks usually happen when a drastic update is needed or in case the community isn't able to agree on the future of the project.