All the planets, except Sun and Moon, have these retrograde periods, but Mercury is most famous for them, probably because Mercury represents our very essential communication abilities. 

While people speak of  Mercury Retrograde periods that screw up computers and television sets, today's astrologers believe the mishaps happen in more personal realms (Uranus is the planet that rules television and computers).  Mercury rules communication, but more  informal communications, like writing, speaking, short shopping sprees and other erranding endeavors.  So, while Mercury is Retrograde, don't give that party,  be extra aware of what you say and what you interpret when chatting with or writing to friends, cut back on errands, expect that the check will be in the mail longer than usual.   Since the car is usually used for shopping and errands,  don't be surprised if the battery wire loosens or the fan belt snaps just when you have to rush out for that one ingredient you forgot to buy. 

The good things to do when Mercury is Retrograde: meditate, contemplate, edit the book/poem/song/essay you've been writing, clean house, talk to your pet, listen to music, paint, catch up on sleep! 

* Encyclopedia of Astrology, Nicholas deVore, Philosophical Library, 1947