This was an upgrade to the Bitcoin network that was implemented on August 23, 2017.
SegWit stands for Segregated Witness. The protocol upgrade was designed to help Bitcoin scale and fix some bugs that represented a risk to its trustless characteristic. SegWit is most known for the way it updates how data is stored on the blockchain.
Its stated purpose was to protect against transaction malleability and decrease transaction times by increasing block capacity. Transaction malleability refers to the possibility that tiny pieces of transaction information could be changed, invalidating new cryptocurrency blocks.
It was also intended to speed up the validation process by storing more transactions in a block.