Thanks for all the reactions.
Nostr is a open protocol/specification it doesn't have a terms of service.
Specific relays, clients or other tools build around nostr like nostr.build may have their own terms of service.
This app would be for hosting your own relay that can store your and your friends notes and other events. There are some public relays, and many relays with a "whitelist" of people who can store there notes on the relay. This relay is unique that it has a invite system inspired by https://lobste.rs/users where only people in the invite tree can write to the relay.
This https://github.com/nostr-protocol/nostr probably explains nostr pretty good. But for the technical specification you can also look at https://github.com/nostr-protocol/nips/blob/master/01.md
If you have any more specific questions let me know:)