In the context of blockchain and cryptocurrency, a soft fork is a change to the software protocol where only previously valid blocks/transactions are made invalid. Since old nodes recognize the new blocks as valid, a soft fork is backward-compatible. This type of fork requires only a majority of the miners upgrading to enforce the new rules, as opposed to a hard fork, which requires all nodes to upgrade and agree on the new version.
In a soft fork, the new rules are a subset of the old rules; any data valid under the new rules is also valid under the old rules. This means that non-upgraded nodes will still see new transactions as valid. However, if a majority of miners have upgraded to enforce the new rules, blocks produced by non-upgraded nodes will be rejected and thus those nodes will follow the chain with the new rules.
Soft forks have been used in many prominent cryptocurrencies, including Bitcoin, to implement new features or updates without causing a split in the blockchain. However, they can still lead to splits if a significant proportion of the network does not recognize or agree with the new rules.