What are sidechains like Liquid?

A sidechain is a separate network linked to Bitcoin. On Liquid you use L-BTC, backed 1:1 by bitcoin, with blocks about every minute and hidden amounts.

The trade-off is trust. Bitcoin moved into Liquid is held by a group of companies that must sign to release it. You get speed and privacy, but you're trusting that group instead of Bitcoin's own rules. Some wallets use Liquid behind the scenes for cheap transfers.