Nostr is an open, decentralised protocol for social communication. Instead of one company owning your account and data, your identity is tied to cryptographic keys that can be used across different Nostr apps.
In practice, that means you can use one Nostr identity to log in, publish posts, follow people and interact across multiple compatible clients.
On Fountain, Nostr can be used to connect your identity to the wider open social ecosystem, making it easier to interact in a way that is portable and not locked to a single platform.
Because it is based on an open protocol, no single app controls the whole network.