A sidechain where you lock up BTC and get L-BTC (Liquid Bitcoin). L-BTC has faster (1 minute) block times. Tokens and stablecoins can be issued on it also. The amount and asset type (Liquid Bitcoin, Liquid USDT, etc...) is confidential. It is less decentralized, and instead of being mined, blocks are signed by a federation of institutions (mostly exchanges) that require a majority.

The biggest usecase is with traders and investors, as it is really good for arbitrage because you can move your coins from one exchange to another quicker and privately, so you are not "announcing" your trade. Since it is mainly used when dealing with centralized exchanges, the fact that it is more centralized than (regular) Bitcoin is not really a problem for this use case.

It iscan open source product from Blockstream. Although it hasnt caught on much, I am happy it is a tool on Bitcoin's belt.

Heres a 3 minute video of Samson Mow explaining it: https://youtu.be/ipBSTuxMlLc