Mercury Retrograde -
So before starting Mercury Retrograde, if you want to know what is Retrograde Planet, please check this link - http://astrosaxena.com/retro.
So, Mercury Retrograde.
Mercury represents communications, business, marketing, logical thinking, quick decision making etc and when it goes retrograde in anyone's birth chart, that person always reflects back at these things. Basically, early in their life, they might have messed up things with their communications or mis-communications, that's why in later part of their life, they are doubly cautious about their communications. These are the people, who will check their email 100 times before pressing the Send button. Actually, it seems all retrograde planets teach us some lessons through our own mistakes in early life, on which we reflect in later life and correct those mistakes.
As Mercury basically represents communication, these people become very cautious about the communications related with the house where Mercury is sitting and aspecting. As it also represents Business and Marketing, they are doubly cautious about the people with whom they are doing business. In early part of life, it may give some impact on communication of person, like this person may not be able to express or communicate himself well enough due to reasons like stammering and all, but as time grows, they surely improve.
So, Mercury Retrograde basically relates to Communication and Mis-Communication.
Hope this helps. Thanks,