SegWit, scheduled to activate on the network very soon, is that it fixes something called transaction malleability. In this article, Im going to explain what it is and why fixing it is important. The Lightning Network is an off-chain micro-payment network. Fixing Transaction Malleability means that the Lightning Network can work smoothly without fixing it. It turns out signatures are the only way in which transaction identifiers can be changed by an attacker. This means that even if the attacker changes the signature, the transaction identifier no longer takes into account the signature.”]
Source: https://bitcointechtalk.com/transaction-malleability-explained-b7e240236fc7