SegWit is a 2017 upgrade that moved signature data (the "witness") into its own part of each transaction. It lowered fees, fixed a long-standing bug, and made the Lightning Network possible. SegWit addresses start with "bc1q."
Go deeperSegWit (BIP 141) activated in August 2017 as a soft fork. It fixed transaction malleability, a bug that let a transaction's ID be changed before it confirmed, which Lightning needed solved. It also replaced the old 1 MB block limit with a 4 million "weight unit" limit that counts witness data at a discount, so blocks can hold more. Taproot later built on top of SegWit.