Lightning Loop is a service that allows users to make a Lightning transaction to an on-chain Bitcoin address (Loop Out), or send on-chain Bitcoin directly into a Lightning channel (Loop In). Loop can help manage channel liquidity, for example, by emptying out a channel and acquiring inbound capacity (or refilling a depleted channel).
Lightning Loop uses submarine swaps to transact non-custodially, meaning that Loop is trustless. Loop Out transactions are batched to save on transaction fees.