A soft fork is a change to Bitcoin's consensus rules that makes them stricter. Anything valid under the new rules is still valid under the old ones, so nodes that do not upgrade keep following the chain.
Examples
P2SH (BIP16, 2012), CLTV (BIP65, 2015), CSV (2016), SegWit (2017) and Taproot (2021) were all soft forks. Many reused opcodes or script forms that old nodes treated as "anyone can spend", and then added rules for them.
The catch
If most hash power does not enforce the new rules, miners could create blocks that break them, and old nodes would accept them. That is why activation methods such as BIP9 wait for broad miner signalling.