The Mercury Retrograde is when mercury passes the earth in its orbit. As it rounds the bend, Mercury slows down and appears to stop (station) and spin backward (retrograde). Of course, it really ISN’T moving backward, but much like two trains or cars passing each other, this creates the optical illusion that one (Mercury, in this case) is going backward.
In astrology, Mercury rules communication, travel, and technology—so all of these areas go haywire for about three weeks. To top it off, Mercury retrograde also has what’s called a “shadow period”, so you may feel the retrograde a couple weeks before and after it actually happens.
(Ok yep, that's why I feel weird already)