The Lightning Network (LN) is a second layer of payment channels on the Bitcoin (BTC) network. These payment channels enable low-fee micro-transactions with improved anonymity.
The largest cost with using LN are when opening and closing payment channels, as each require an on-chain BTC transaction. The price of which depends on current congestion, but can range from ~$0.10 to $20, or even more to open and again to close the channel. Once the payment channel is open, the payments within the LN are very cheap and fast.