The Lightning Network (LN) is a second layer scaling product on the Bitcoin (BTC) network. The LN is a series of payment channels which enable low-fee micro-transactions with improved anonymity.
The largest cost of using the Lightning Network are the transactions to open, fund and close payment channels. Each action require an on-chain BTC transaction. The fee of each transactions depends on current congestion, but usually ranges from ~$0.10 to $20, sometimes more. Once the payment channel is open, the payments on Lightning Network are fast with low fees.
It will be interesting to see if this gains adoption, or if the cash use-case moves to other chains focused on cheap L1 fees.
* All terms and definitions may update as the Cryptionary improves.