A soft fork is a software fork in a blockchain protocol where nodes that support the older software version continue to be able to process new transactions. This means new rules can be introduced while maintaining compatibility with the older version of the software.
It is key to understanding how blockchain protocols, such as Bitcoin, are able to upgrade and adapt over time. Soft forks differ from hard forks in that all nodes that support the old version of the software can still process new transactions. This means that miners running the old version of the software are still able to mine blocks and add them to the blockchain, and all nodes are still able to validate transactions and blocks.
A hard fork, however, would require all miners and nodes to upgrade to the new version of the software, otherwise they would not be able to participate in consensus. Soft forks enable developers to introduce new features without getting all nodes on board with the new changes.
This makes them useful for introducing backwards compatible minor changes to a protocol, allowing the blockchain to grow organically and make improvements to the system over time. They also make it easier for different nodes running different versions of the software to communicate with each other.