Discord is a chat application. It exists of a lot of different servers where a lot of Steemians hang out. You can call it 'the behind-the-scenes' of SteemIt.
Most communities have their own server. In their posts, they often add a link to join their Discord server.
Let me invite you to a server I have set up for the newcomers I'm helping out. Clicking this link will bring you to the Redfish Rockets server:
I hope I will be online when you arrive there (because I'm on and off all day long), but once you've created your account and have arrived in the server, leave me a message in the #redfish-questions channel. We'll take it from there.