I thought “Communities” were already used to group rooms?

Anyway I searched more for these “Spaces” and I found this:

Spaces are particularly interesting because they open up the possibility of Matrix being more than just a big flat namespace of conversations: instead they provide a global fully decentralised hierarchical filesystem, complete with decentralised ACLs, allowing users to publish and curate an arbitrary taxonomy of whatever data they choose (be it real-time conversations, history, data streams, files, objects, etc). This has potential to flip Matrix entirely on its head: Spaces could become the main backbone of the protocol, with chatrooms being mere leaf nodes in a giant tree of collaboration.