Of course, that does not really happen, but the optical illusion is there. Mercury is the fastest moving planet that we know of in its orbit around the Sun: it takes approximately 88 days. In astrological terms, Mercury represents our capacity to communicate and think, thereby "ruling" (or 'sponsoring') all types of mental activities and delivery of thoughts: written and verbal, as well as contracts, signings of such, and the industries that provide these services (phones, computers, electronics).
A retrograde indicates an internalization process; influences and actions that would be energized for the planet(s) that are in a retro cycle are slower to manifest: they are more ‘inside than outside’ and take longer to complete, comprehend, or activate. As I post this, Mercury has been retrograde since August 20th; it will go direct on September 12th.
In the case of Mercury retrograde, people often become forgetful. When it goes retrograde for its phases as the Earth briefly overtakes it, all types of communication go 'unsteady.' It is often observed that papers are misplaced, documents and important transactions are not processed as planned, people often complain that they can't focus or think clearly, or other matters of sharing information are cast adrift until that point when Mercury once again regains its position as leader and overtakes the Earth. As I wrote this, Mercury was indeed retrograde--so I can relate to anything during this time that went haywire and all lost-and-off-the-map. As it happened, I misplaced two vital documents on the same day—and they’re still hiding from me in plain sight.
As noted above, it's about a 4-6 week process that happens about three or four times a year. An astrological calendar or farmer's almanac will show this; sometimes I have seen this on calendars I've received from pharmacies or other manufacturers. There is also a book published for a 50-year cycle available in bookstores called an ephemeris; it shows the retrogrades and planetary changes on a spreadsheet format with symbols for each significant solar system object.
Has this any good potential aside from feeling as though one has lost his or her mind for a temporary period of time or that it's not appropriate to sign that important paper? Yes: some people find they can actually focus more clearly on a single idea during Mercury retrograde. Some writers (myself included) and people who may be considered hyperactive or ADD may find they can actually do more work on a regimented schedule during a Mercury retrograde. I have found some of my better writing efforts have come during a time frame like this. On the other hand, I also bounced a series of bank transactions the day that Mercury went backwards, a set of events that cost me quite a few dollars and grief! However, I certainly can't blame the planet: it only indicates a potential set of activities and actions, just like any other planet or heavenly object that we use when interpreting astrological charts.
Therefore, it's not an excuse to say "Omigod, Mercury is retrograde: I've lost my mind and forgotten to pay the bills, am over-drawn at the bank, forgotten my password sequence, haven’t fed the dog/cat, didn’t empty the garbage, have no gas in the car, lost the keys, and don’t a clue as to what I need to write for my college paper that's due tomorrow!" But in all good fun and effort, it might be a proper time during a retrograde to pick up that unfinished book, paintbrush, project, or other incomplete hobby and see how much more focused one can be as he or she feels.
Just put your mind to it.