The term "Retrograde" means to back-track, or retrace steps. All planets in our solar system have Retrograde phases except for the Sun (because Earth orbits it), and the moon (because it orbits Earth). Mercury is closest to the Sun, so it completes an orbit around the Sun once every four months, give or take. During every circuit, it "appears" to move backwards (from Earth's point of view) for about 3 weeks. Mercury rules communication (electronic and inter-personal) and transportation.
When Mercury shifts into a Retrograde phase, it causes friction with the flow of information and modes of travel; things occur more frequently, like: missing phone calls, texts or emails gone awry, telephone and computer snafus, automotive glitches, travel detours and delays abound. Retrograde is all about "RE-do," so it's best to take care of old business during this time, because attempting to begin new projects have a tendency to require a RE-attempt later.
Knowing where Mercury makes this dance step each time it occurs, in comparison to your personal birth-chart, can help prepare for what area of your life is most likely to expect chaos -- and how best to avoid any potential potholes in your path ahead.
I'll be teaching a class on this topic Friday, July 21 at 6:30 - 8:00 PM at the New Age Center in Derby. If you're interested in attending, please bring a printed copy of your birth chart if you have one. If not, tell us your DOB, place (city/state) and time of birth -- and we'll accommodate you.
Hope to see you there!