The lightning network is a second layer technology applied to Bitcoin (and other compatible protocols) that uses micropayment channels to scale its blockchain’s capability to conduct transactions more efficiently. Transactions conducted on lightning networks run on its own 2nd layer solution, and are faster, less costly, as well as more readily confirmed than those conducted directly on the blockchain (i.e., on-chain).
By taking transactions away from the main blockchain and making them off-chain, the lightning network was designed to de-congest the blockchain and reduce associated transaction fees. The lightning network can also be used to conduct other types of off-chain transactions involving exchanges between cryptocurrencies.
For example, it is helpful for facilitating atomic swaps that enable one cryptocurrency to be exchanged for another without the involvement of an intermediary, such as cryptocurrency exchanges.