Lightning Network is a 2nd-level blockchain protocol that uses smart-contracts for achieving high-speed transactions. The main principle is using of payment channels and every channel is a wallet with some sum of crypto. All transactions between members are going strictly without data recording to blockchain. Shortly, payment channel is a safe and crypto is available only for some blockchain members during transactions (protection is guaranteed by private keys).
Due to the fact that this channel is built on a network principle, even if the two persons who must complete the transaction do not have a common open channel, they can be connected through some other participants (i.e. participants can exchange transactions with every other member through nodes – and participants also are nodes.)
But, there of course are problems here – the danger of network centralization.