An alternative chat protocol. You can self host it or join other self hosted instances, same as lemmy and mastodon.
It works well and comes with encryption, group chat, video and voice chat and maybe a dozen different clients. No single company can shut you down in opposition to discord.
Edit: matrix also has bridges to discord so you wont likely loose your friends from discord. The difference to an alternative discord client is that it runs on a completely different protocol with no chance for discord to shut down your matrix account since its not on discord.