It's like Slack for the gaming world, either you create a group or get invited to one and you use the skype-like VOIP functionality to talk to others in your group. It doesn't use any of the games per se although I think you can setup games to be launched from it. It's just a way to have live chats with people you want to play games with.
You set up a server, add channels for text/voice, &c, add roles, rules, color things, add pictures, invite people using either a permanent link or a timed one.
Discord is awesome... It's like Skype, voice chat program with servers and the servers have their voice+text chat channels. You need to be invited to the server (click on an invitation link). I heard Splatoon 2 peeps have some discords or something... especially premades. cool stuff