Mercury was the Messenger God - running back and forth between the God realm and the Human realm, bringing new insights from God to Humans and reporting goings on from Humans to Gods.
In Astrology, Mercury represents the thinking function. It is how we take in information from the world around us, how we digest and integrate that information and how we communicate it back - educating others of our unique point of view.
So when we have Mercury in an Air Sign (Air signifying intellect) our mind orients around logic and conceptual ideas. If we have Mercury in an Earth Sign we take in information from our five sense and we tend to put our trust in matters of the physical world. Concepts are less valued. If we have Mercury in a Water sign, our feelings rule our mind. We trust what we feel. Our thinking can become flooded when our feelings run amok, yet when we are clear, we have a profound capacity for empathy. When Mercury is in a fire sign our mind follows the ignition of our passions and we have natural insight. We communicate from the warmth in our heart.
Mercury is the closest planet to the Sun and therefore it is common in Astrology charts to see Mercury conjunct (next to) the Sun. These are folks who can’t seem to get out of their heads! (And we all know a few of them!)
Also, about three times a year or so, Planet Mercury appears to be moving backward in the sky. This is known as Mercury Retrograde. Though Mercury is not actually moving backwards - this is simply the illusion from Earth as our revolution around the Sun mis-align with Mercury - Mercury Retrograde symbolizes a time for retreat.
The common lore around Mercury Retrograde is that communication becomes confused and our electronics go on the blink. It is definitely easy to notice both of these under a Mercury Retrograde. However the true purpose of Mercury Retrograde is for retreat - a time to step back from the world and process all we have learned and taken in from the world in the most recent months. It is time to spend time with those people we are the most comfortable with and we have the easiest time talking to. If we put our emphasis on that - deeper communication with our favorite people - and less on communication mishaps, we will have a more enjoyable time with Mercury Retrograde. I allow the communication mishaps and the electronics on the blink serve as a reminder to relax and retreat and maybe connect with a special friend.