Lightning Loop is a non custodial service offered by Lightning Labs to bridge on-chain and off-chain Bitcoin using Submarine swaps.
You have a channel with a local balance amount and you want to gain inbound liquidity.
Your node pays a lightning invoice for the amount requested via the loop service. This moves the local balance, for the amount paid, to the remote side of the channel.
Loop service then sends you a payment on-chain for the amount same as the lightning payment minus the fee.
Final settlement occurs when your node sweeps the on-chain payment and the loop server settles the lightning invoice. You receive the payment on-chain in your wallet and also move local balance to the remote side of the channel, gaining inbound capacity.