It is a second layer protocol on top of Bitcoin that defines rules and contracts which enable the fast, secure, private, trust-less and permission-less transfer of Bitcoin.
Users of the Lightning Network are able to transfer Bitcoin among each other at virtually no cost and in real time.
There is no necessity to wait for block confirmations for payments.
Once a payment arrives, it is final and cannot be reversed.
Like a standard Bitcoin transaction a payment on the Lightning Network can only be refunded by the recipient.
While all Bitcoin transactions are stored in the blockchain where they can be tracked the payments on the Lightning Network are off-chain and offer more privacy than Bitcoin payments.
Due to the used onion routing, which is also used in tor even the nodes that are involved in forwarding the payment from the sender to the recipient do not know for whom they deliver the payment.
A person who wants to earn interest on their bitcoin without the risk of lending them to other people could decide to set up routing nodes on the lightning network.
By providing liquidity to the Lightning Network the routing capacities will be increased offering the chance to earn routing fees on the owned bitcoin.