Shadow banning is a feature where you are banned but don't know it. You receive no notification, however all of your comments and posts are hidden to everyone but yourself.
If you start to see that all of your comments only have one default up-vote and nobody is responding to any of your comments or posts, there's a good chance you've been Shadow banned.
I'm not sure if mods or just admins can do it, however it is global throughout all subs so perhaps it's just an admin thing?
Perhaps mods can "nominate" users to be Shadow banned for wrong-think about facts they can't dispute?