Matrix is like a Fediverse thing for messaging, like a Discord or Slack... But distributed, encrypted by default, has many federated instances, etc. You'll use it for real-time-ish messaging, presumably with the rest of the mod team.
You need to pick a client for Matrix... Kind of like how apps like Jerboa and Connect are clients for Lemmy. Element is a popular client for it, and the one I use. You should be able to go to Element, select an instance to make your account with (like you've done with Lemmy), and join the "room" that they sent you, which may be on another instance, but it'll work fine anyway (also like Lemmy).
When I search "Maxtrix malware," I get some results for a ransomware virus, which is probably true...? But unrelated to the Matrix messaging protocol.