Lightning is a second layer on top of Bitcoin designed for small, frequent payments. Instead of writing every payment to the blockchain, nodes open a two-sided payment channel, settle thousands of transactions off-chain, and only write the final balance back to Bitcoin when the channel closes.
For a Nostr user, Lightning is what makes zaps practical. Sending one sat costs effectively nothing, arrives in a second, and cannot be reversed by an intermediary. The whole zap mechanism would not work on base Bitcoin alone: a one-cent tip would cost more in fees than the tip itself.