Discord has 5 status icons that are pretty simple representations of user status. They are as follows:
Green = Online
Yellow = Idle
Red = Do Not Disturb
Purple = Streaming
Gray = Invisible/Offline
Keep in mind that you only get put into idle status when you've been away from your device for a while- similar to being moved to an AFK (away from keyboard) voice channel. And while Offline doesn't have a dot, you'll see the user's name and avatar grayed out pretty thoroughly.