A Soft Fork is a modification of the software protocol where only previously valid blocks/transactions are made invalid. Since old nodes will recognize the new blocks as valid, a Soft Fork is backwards compatible.
This type of Fork requires a majority of miners to upgrade to apply the new rules, as opposed to a Hard Fork which requires all nodes to upgrade and agree on the new version.
A Soft Fork can also sometimes occur due to a temporary discrepancy in the blockchain when miners using non-upgraded nodes violate a new consensus rule that their nodes are not aware of. For this reason, it is still important for node operators to keep abreast of the actions that need to be implemented for blockchain upgrades.
Several Soft Forks have been used on the Bitcoin and Ethereum blockchains to implement new and improved features that are backwards compatible.
