It means you think your post are being made public but there not either being posted at all or only to your closest friends who have not submitted complaints about your account recently. For other purpose the website/app seems to work perfectly and does not mention you being flagged, sometimes it's hard to tell, and even include fake view counts, so your not suspicious.

Reddit also has shadow-banning, apparently to stop bots and spammers.

There are two things wrong with that idea. First, the false-positive rate was too high - I created more than 10 personal accounts over almost a year, and they were ALL shadow-banned, often after one post. Second, if I was automating spam bots I wouldn't care if one was shadow-banned. It's not hard to automate a test for being banned, so just create a new bot and move on.