What does it mean if Mercury is totally in retrograde?

Answer 1

It means that it appears that it is moving backwards. (In Astrology, it also means that things surrounding communication will be messed up and you should be careful what you say, how you communicate and not make any big decisions during that time.)

Answer 2

It means Mercury is moving backward against the background of fixed stars compared to the direction is usually moves. This is only apparent motion as seen from Earth of course; an illusion caused by the combined motions of Earth and Mercury in their respective orbits. Mercury never actually backs up in its orbit.