Also called, “Chain Split” or a “Hard Fork”, a “Fork” is what happens should the blockchain diverge into two potential paths or a change in protocol. What becomes of the single blockchain is that it is split in two leaving it to run at the same time. An example of a Fork or Chain Split is Ethereum and Ethereum Classic.
The radical change that takes place within the protocol makes the transactions that been previously labelled as invalid, valid. It also goes the other way around. What is required of all nodes or users is to upgrade to the latest version of the protocol software.
Forks can happen in any crypto exchange platform, not just Bitcoin. This is because blockchains and cryptos actually work in similar ways regardless of the platform.