Matrix is an open-source standard for decentralized communication. It is constantly evolving to the requirements of modern solutions keeping in mind the privacy of users.
You will find Matrix clients as a replacement for Slack, Microsoft Teams, and several other popular communication platforms....
Not sure why Signal gets all the attention when Matrix is much more successful at spreading open source messaging and is also decentralized.
Signal has a lot of things going for it Matrix doesn't have... more convenient, simpler, easier to use, more focus on privacy, better calling. Matrix does some things Signal does, but they have different feature sets really. Matrix being free software, unfortunately, doesn't matter to most people. Matrix also arguably isn't really about security and privacy. It's more of a universal chat platform, so they don't compete directly.
The fact the Matrix is usually marketed as a platform for secure and private communications is somewhat difficult to understand.
I don't think Signal is much easier than Matrix. This is why I'm so excited about Telegram. Telegram's an easy to use and feature rich chat app that also has open source clients and decent Linux support. It's great for using with non-techie friends. Matrix is great for using with techie friends. Signal is somewhere in between, and doesn't really fullfill either role.