A fork is basically an alteration of the current Bitcoin protocol changing the rules. As an example, if you were playing an online game and wanted to change the rules, all other players will need to agree on the rule change. If successful, the changes are implemented and the game continues as normal. However, if the consensus was not large enough then two versions of the game are created (version #1 = old rules, version #2 = new rules). This is now a fork in the game. The same analogy applies to Bitcoin’s code.
Post forking there is an “original Bitcoin” and a “new Bitcoin.” For example, Bitcoin Cash changed the block size from 1 MB to 8. The supporters switched to a new coin “Bitcoin Cash”, and those who preferred the original rules kept using the original Bitcoin.