Matrix is an open source, federated instant messaging network. Being federated means that the servers are hosted by multiple different people, and are all interconnected. Its ecosystem is made up of open source, community driven software, meaning that the servers and programs you interact with are transparent in what they do.
Matrix has instant messaging, group chats, voice/video calls, screensharing, and much more. Its all the things you'd expect, and on top of that it features optional end-to-end encryption.
Its optional e2e encryption means that you can make sure private conversations stay private, no snooping from the Matrix network or anyone in between. Being federated means that the network is decentralized, there's no single point of failure that brings down the entire service. No one person owns Matrix either, that combined with the community oriented nature of Matrix means that your data isn't being sold, and features aren't pay-walled. You also have complete freedom in what client you use for Matrix, and what server you register your account on.
You first select a server to create your account on, this is your homeserver. Any trustworthy and well performing server will do. You often sign up for a server through a client, from which you have many options to choose from. There are browser clients, desktop clients for many operating systems, and mobile clients for phones. Once you sign up, you'll be asked to store a recovery key. Keep this safe and secure, its how you recover your encrypted messages if needed.
There's plenty of communities that live on Matrix. You can try joining some that you like. If you're happy with your experience, you could let your other online friends know about it. They can try it for the first time, and those who enjoy it could talk with you there instead of on your current platform.