What Is Mercury Retrograde?

I see a lot of cope year round on social media about people not being themselves, or feeling off/tired during various astrological phenomena. Recently I saw some mercury retrograde posts and semi looked into it, but SEO is a hell of a thing, and all of the best click bait and crap is all over the front page of various searches.

From what I gathered it’s when Mercury slows its revolution around the sun? Like it slows down quite a bit for a period of time. Why would that affect anyone here on earth? Is this basically more cope like zodiac signs and other memes?
Lurking since '05 - the days of VS gorilla/cloud/goku threads