What makes an actor A list, B list etc? What the hell does that even mean?

"A" list are actors who tend to be respected by the general public and their peers. They've also acted in well known movies. Example: Tom Hanks

"D" list are ones that haven't done much acting and tend to not be respected too much by others. And the acting that they did was in not too good movies or shows. They tend to also have problems getting work. Example: Dustin Diamond