Fork refers to the change made to a blockchain causing the blockchain to have two different versions. These are usually referred to as blockchain updates effected to improve security, eliminate bugs or enhance scalability in the blockchain.
Fork is of two types: Soft and Hard Fork. A soft fork refers to a small changes or updates made to a blockchain and does not cause dramatic changes in the nodes’ interactions. A hard fork is a radical change that causes the blockchain to change or improve dramatically and requires the nodes to be upgraded to the new version.