Mercury represents intellect, and the house where it resides in a horoscope defines the area where the natives use their intellect. This is the area of life where you find it the easiest to express your ideas and thoughts. It represents what you think (intellect) and talk (communication) about the most. You mind is actively involved in the area of your Mercury house, thinking, cataloging, rationalizing, disposing and expressing what you experience.
Mercury is the closest planet to Sun, so it is under the strong influence of this mighty celestial body. You will always find Mercury, together or on either side of where Sun is placed. Sun represents the soul and individuality, which uses mind (represented by Mercury) to express itself in the best way.
When mind and individuality are in harmony, meaning when Sun and Mercury are together in a house, it forms Budh Aditya Yoga. If Mercury is one house ahead of Sun, the person turns out to be cunning and uses their intelligence manipulatively. And when it is behind Sun, the native uses their mind constructively but has to put in more efforts to boost intellect. Mercury, representing consciousness is thus the mediator between soul, and how it manifests itself in physical expression.