Segregated Witness (SegWit) refers to a process to reduce the requirements for processing Bitcoin transactions by allowing information to be processed more efficiently to speed up the network. Specifically this refers to digital signatures used to verify transactions, which take up 65% of a transaction’s information. Blocks are mined every 10 minutes with a maximum load of 1MB of data per block, so verifying these signatures was slowing down transaction speeds until the upgrade was introduced in 2017 by Dr Pieter Wuille, after first proving its effectiveness on the Litecoin network.
Signatures are now processed separately towards the backend of each block. An advantage of this reorganisation means that the 1MB limit per block was increased fourfold to around 4MB, allowing for more transactions to be processed per 10 minute block. As the Bitcoin network has grown over the years, this upgrade has been key in allowing Bitcoin payments to scale.