Segwit is short for Segregated Witnesses which is a soft fork upgrade to the Bitcoin network that was activated on August 24, 2017. The actual technical workings are fairly complicated but well documented for the curious.
SegWit transactions are more efficient than other legacy transaction types. With SegWit enabled by default, you pay less in miner fees when transacting with Samourai Wallet and benefit from cheaper transactions.
Samourai wallet was the second mobile wallet to implement the Segwit upgrade and native Segwit is enabled by default in Samourai Wallet.
Samourai supports both wrapped Segwit (BIP49 addresses starting with 3) and native Segwit (BIP84 addresses starting with bc1).
Learn more about how Segwit works with this video.