Matrix is an open standard for secure, decentralized, real-time communication. Think of it as an open protocol for messaging, similar to how email works - anyone can run their own server, and all servers can communicate with each other.

https://matrixdocs.github.io/docs/getting-started/introduction

I personally recommend using Element as a client.

The Blahaj Zone provides a public Matrix server you can join (see the sidebar), here’s the link for convenience: https://matrix.to/#/#blahaj:chat.blahaj.zone