Lightning Loop is a non custodial service offered by Lightning Labs to bridge on-chain and off-chain Bitcoin using Submarine swaps.
Your outgoing capacity is depleted and you want to regain it without opening new channels.
Your node sends funds on-chain to loop server to be swapped with off-chain liquidity.
Loop server sends equivalent funds off-chain to your node by making a lightning payment to you.
You send the payment on-chain from your wallet and also move remote balance to the local side of the node, gaining outgoing capacity.