Online status is the default status set by Discord. When you are online the green dot will come next to your avatar pic. Which show you are online can be reached easily now.
02. Idle Status
1. Idle status shows that you are away and not active right now. it's just like the Discord is open on the screen but there is no activity for some period
Do Not Disturb (DND) Status
1. DND Status allows you to stop notifications and work in peace. when you enable the DND mode the red dot with a line comes next to your avatar
04. Invisible Status
Invisible status allow the user to stay invisible and have access the all the Discord features as normal. The Invisible mode helps you to avoid people and stay active and do your desired work.