Policing online communities can be extremely difficult, whether we're talking about over 2 billion Facebook users or a small forum dedicated to antique clock repair. Just ask Mark Zuckerberg (like Congress did).
One of the many ways to pacify problematic users is to employ a tool called a "shadow ban." The term originates from a time where internet communities primarily existed in individual, isolated web forums - rather than outright ban a user, a forum moderator would "shadow" ban them. They could read posts, and even make their own as usual. But nobody else in the community could see their posts, or even know that they tried to say something.
It's as if they became a shadow. Spooky!
On Thursday morning, President Trump tweeted, "Twitter 'SHADOW BANNING' prominent Republicans. Not good. We will look into this discriminatory and illegal practice at once! Many complaints." So, did Twitter shadow ban prominent Republicans, as Trump alleges?
It doesn't appear that it did, except in the most technical sense.