Lightning Loop is a non-custodial service by Lightning Labs providing submarine swaps. It allows you to "loop in and out".
Initially (March 2019) Lightning Loop focused on the ability for people to receive with what we call Loop Out.
Lightning Loop Out allows users to increase their receiving capacity by offloading their funds from the network while keeping channels open.
This will come in a future release
Loop In enables users to refill their Lightning channels with on-chain bitcoin from wallets or exchanges