The moon is part of "the lesser light to rule the night" in Genesis 1.
Physically its a big rock that orbits earth roughly 375,000 kilometers away. It has no light of its own, but does reflect sunlight to the earth. It is used to distinguish between months.
Metaphorically the moon is the next most powerful authority within a kingdom, typically the kingdom of Israel. For Physical Israel this was the High Priest. For Spiritual Israel, the church, Christ is both King and High Priest so there is no moon.