The Moon is the second biggest thing in the Earth's sky, after the Sun, not counting manmade objects (like airplanes, helicopters, mechanical birds, or catapulted children). It's a large orb that orbits our planet, and always shows the same face to us, it doesn't rotate. Nationalistic myth in the United States claim that we sent astronauts there, but they didn't do much except play golf and hop around a lot.
The "moon" has a lot of myth and lore associated with it. Full moons in particular, which occur when the entire face of the moon that faces the earth, is lit by the sun, and hence appears as a full circle from the earth. The moon is also frequently associated with women because it's cycle is approximately the same as a woman's menstrual cycle.