It's a lightning invoice receipt. But in basic form, it's allowing users to easily send sats via clients which helps perpetuate a value for value system. If you set up a wallet supporting zaps (NIP57) on your nostr profile you can start utilizing them. WoS supports it, as well as Zebedee now (as wel...

There are many smarter people than I on here that could elaborate if not ammend my answer. But here's the github: https://github.com/nostr-protocol/nips/pull/224

We'll show you if you setup a lightning address instead of a lightning invoice on your profile.⚡⚡