Casual communication is important, the most popular ways to do that in my circles are Discord and Telegram. Discord in particular quite overzealously shares and scans through the conversations you have as well as collecting data on you which is then shared with whoever is willing to pay. When an administration change can result in you being labeled as a terrorist, would you want all your casual communications on a platform that will gleefully share what they know about you with the people who want you dead? Are you comfortable having your conversations used as AI training data?

Matrix is comparable to Discord in function- the main caveats being that it is decentralized and supports end to end encryption. Meaning anyone can host a comparable space to a discord server and host accounts. Accounts can move around between servers, just like on discord. So, my account levee:canadathistle.com can join a server on mozilla.org example:mozzilla.org and user:mozilla.org can join my canadathistle:canadathistle.com server. As a result, conversations that happen on a server stay there and monitoring and sharing can't occur the way they do on discord. Matrix also supports end to end encryption which enhances the security of conversations. All this being said Matrix is by no means perfect and I do not pretend that it is, but it is significantly better than discord.

You need an account and a client.

Compare this to email, where you make an email account at gmail, or your school etc. Then you have an email app which you use to access your email account. While I will make accounts on my server for people I advise making an account on a more stable service- I'm just a silly girl and prone to making mistakes :3

Cinny is my favorite client, it also supports making accounts. Here is how to use it