To start from the beginning of an explanation, please read that article: Motion of the Planets
Mercury rules Gemini and Virgo; it is exalted in Virgo, has its fall in Pisces and in detriment in Sagittarius.
In addition to signs of rulership, the planets manifest their power in signs of exaltation. Exaltation indicates a strong desire of the native to manifest the principle of the planet in the outward world, which is not always possible to accomplish. The main reason for this is the lack of desire to manifest the principle of the planet in life. Without the active use of the energy of the planet, there will be no realization.