Definition
A soft fork is a change to the Bitcoin protocol that makes the rules stricter. Blocks produced under the new rules are still valid under the old rules, meaning non-upgraded nodes continue to function (though they may not enforce the new rules). This backward compatibility makes soft forks less disruptive than hard forks.
Notable Bitcoin soft forks include SegWit (Segregated Witness) in 2017, which changed how transaction data is stored, and Taproot in 2021, which improved scripting capabilities and privacy.
In Simple Terms
A backward-compatible protocol upgrade. Old nodes still accept new blocks without upgrading.
A backward-compatible protocol upgrade that tightens the rules. Old nodes still accept new blocks, but new nodes may reject some old-style blocks.