A fork is a change to Bitcoin's rules. A soft fork tightens the rules and old software still works with it. A hard fork loosens them and everyone must upgrade, or the chain splits in two.
Go deeperSegWit and Taproot were soft forks, so nobody was forced to upgrade. Bitcoin Cash came from a hard fork in 2017 and became a separate coin. The word "fork" is also used for a project that copies Bitcoin's code to make a new coin.