SegWit is short for Segregated Witness, which is a BIP (Bitcoin Improvement Proposal). This particular BIP was designed to make Bitcoin transactions less affected by pressure on the network. Previously, when you changed the “witness” information (meaning the signatures) on a certain block, it would also change the so called transaction ID of such block. This would also change the block’s subsequent hash. By the introduction of SegWit, signatures can be segregated (thus “SegWit”). This, in turn, means that block sizes can be smaller and Second-Layer Solutions can be supported.