A fork of a crypto coin is basically taking the codebase of an existing project and modifies it a little or even a lot and then you basically have a "new" crypto project that is independent from the original one even though it is maybe 99% identical with it. There are many forks in the crypto space. We already have Ethereum Classic for example, we have Bitcoin cash and many other forks.
That proof of work Ethereum fork was created by big miner because obviously they have lost a lot of potential profit with the switch of ETH to proof of stake. That is why they tried to introduce a version of Ethereum that is still proof of work, so they can continue to mine. It seems like that project is not doing so well though.