The Moon doesn't generate its own light - it reflects the Sun's light. In astrology, this symbolizes the Moon's role as the reflector of consciousness, emotions, and the subconscious.
Just as the Moon reflects solar light, your Moon sign reflects how you process and express the energy of your Sun sign. The Moon is responsive, receptive, and reactive.
During a new moon (when the Moon is between Earth and Sun), we can't see it because it's not reflecting light toward Earth - symbolically a time for new beginnings and planting seeds.