The Moon is the most visible symbol in the solar system, which reflects the light from the sun, it represents the feminine side of us, the unconscious aspects of our dreams, memories, moods and mostly our emotions. The Moon Sign ruled by the element of water (our emotions) which effects the tide of the sea.
For example when the moon is at is fullest, the tide is usually at its highest, which brings out our emotions more, and bringing things into fruition.
The Moon is the ruler of the sun sign Cancer, which again is a very sensitive sign, and has its nurturing qualities to it.
The Moon is the closest planet to Earth, and manages to zip around all of the Zodiac signs within 28 days, it also spends around 2-3 days in each sun sign, so depending on which sign the moon is currently in, explains alot about our current emotions.