SegWit, 2017
Segregated Witness moved signatures, the witness, to a separate part of the transaction. It fixed transaction malleability, a flaw that let third parties alter a transaction's id before confirmation and that blocked protocols like Lightning. It also replaced the 1 MB size limit with a weight limit of 4 million units, where witness data counts less, which raised capacity.
Taproot, 2021
Taproot activated in November 2021. It brought Schnorr signatures, which can combine the keys of several people into one, and a way to hide alternative spending conditions in a tree of scripts. Only the condition actually used is revealed when spending.
The result: a multisig or a complex contract spent in the cooperative way looks exactly like a single signature payment. That is cheaper and more private for everyone using it.