Segregated Witness (BIP141) is a soft fork that activated at block 481,824 on 24 August 2017.
It moved signatures (the witness) into a separate part of the transaction. That fixed transaction malleability, which made the Lightning Network practical. It also replaced the 1 MB size limit with a 4 million weight unit limit.
Activation followed years of debate, the block size war. Users backed it with the BIP148 "user-activated soft fork" campaign.
Takeaway: SegWit added capacity and enabled Layer 2 without a hard fork.