A soft fork (softfork) is a change to the cryptocurrency protocol wherein only previously valid blocks and transactions are made invalid. Since old nodes will recognize the new blocks as valid, unlike a hard fork, a soft fork can still work with older versions of the protocol. If a wallet wants to support new soft fork rules it has to be upgraded.
See also bitcoin wiki page
Soft forks in Bitcoin:
- Introducing the limit of 1MB for a block was done through a soft fork
- The pay-to-script-hash function, which enhances the code without changing the structure was also successfully added through a soft fork.
- SegWit was implemented in Bitcoin protocol using a soft fork in BIP141.
See also this article for more information about differences between soft forks and hard forks.
UASF vs MASF
When a majority of miners upgrade to enforce new rules, it is called a miner-activated soft fork (MASF). When full nodes coordinate to enforce new rules, without the support from miners, it is called a user-activated soft fork (UASF).