Matrix aims to be a protocol for mostly real-time things like chat and voice/video calls.
It has a data structure called rooms (think: chat rooms) that are spread out across multiple servers and the servers synchronize the content between each other. While ActivityPub (what most of Fediverse uses) is much simpler and just list posts adding API for interactions. Matrix aim to be a fabric to build decentralized alternatives of Discord, Zoom, WhatsApp, Google Classroom, Jamboard, Google Docs, etc.