The Lightning Network is a "second tier" payment protocol that operates on blockchain (most often Bitcoin). It allows instant transactions between participating nodes and has been touted as a solution to bitcoin scalability issues.
The normal use of the Lightning Network consists of opening a payment channel by funding transactions to the relevant blockchain, followed by making a number of Lightning transactions that renew the temporary distribution of unallocated channel funds to blockchain, optionally followed by closing the payment channel by broadcasting the final version of the transaction to distribute channel funds.