If there is a particular Discord server that you would like to mute, start by right-clicking on the server icon for it. Then click Notification Settings. You'll find a Mute [Server Name] option, which is all you need to stop getting notifications for every message. If you want to take it a step further, you can also select to Suppress @everyone and @here, which will mute notifications from those tags. You will still see a red notification badge still, however.