A fork in cryptocurrency occurs when software updates need to be implemented. The more far-reaching the changes, the more controversial the fork.
As with any metaphorical fork in the road, at some point there needs to be a decision on the direction forward for the cryptocurrency. Consensus must be reached among the community of a specific cryptocurrency before changes can be implemented. If developers in the community are unhappy with the confirmed changes, they can jump ship and create a new cryptocurrency. At this point, the blockchain that the cryptocurrency operates on will duplicate and split, and the two communities go their separate ways to implement their own design solutions. This is called a hard fork.
Bitcoin Cash is one example of a hard fork. It was a result of the so-called ‘SegWit’ Bitcoin upgrade in 2017, in which developers who disagreed with the SegWit upgrade created their own Bitcoin Cash blockchain which followed the rules they thought should apply to Bitcoin, while those happy with the changes made in the SegWit upgrade continued on the original Bitcoin blockchain.