What is a Monero 'Hardfork'?
A Hardfork, in relation to blockchain technology, is a change to the protocol.
It can be compared to a software update, it upgrades the network and improves security and privacy.
A hard fork requires that all nodes or users upgrade to the latest version of the protocol software.
Since it is a permanent upgrade, any nodes running the previous version will no longer be accepted by the new version.
This is where the "fork" comes in. One path will follow the new upgraded blockchain, and the other will continue down the old path.
The latest hardfork was July 2022, v5.