“Liquid Bitcoin (L-BTC) is a special type of asset on the Liquid sidechain, with a supply that is verifiably backed 1-to-1 with bitcoins (BTC) held by the Liquid Federation on the Bitcoin mainchain.”

https://help.blockstream.com/hc/en-us/articles/900001408623-How-does-Liquid-Bitcoin-L-BTC-work-