Plain meaning
Start with the shortest useful explanation before going deeper.
A Layer 2 payment channel network built on Bitcoin that enables near-instant, low-cost transactions by conducting most activity off-chain. Two parties open a channel by locking BTC in a multisig transaction, then exchange signed commitment transactions off-chain; only the opening and closing transactions are broadcast on-chain. Payments can be routed across multiple channels using Hash Time-Locked Contracts (HTLCs), enabling trustless multi-hop transfers.