You can think of Yjs as a protocol for data synchronization. It gives you a way to describe a JSON-like data structure (i.e. nested lists and maps), and keep them in sync across multiple devices.

Yjs itself doesn't provide a platform, but it's an open protocol so there there are service providers (like ourselves[1]) that offer Yjs backends as a service (other notable providers are TipTap/Hocusocus and Liveblocks).

[1] https://jamsocket.com/y-sweet