The moon orbits the Earth, causing it to appear as though it's shrinking or growing. When it appears to be shrinking, it's called waning and when it appears to be growing, it's called waxing. The moon sometimes is more than half illuminated, called gibbous ( :moon: ) and sometimes less than half illuminated, called crescent ( :waning_crescent_moon: ). Occasionally, it even appears red in what we call a lunar eclipse. It can also cause the sun to seem to disappear from the sky, a solar eclipse, by obscuring the sun.
The moon and Earth are both celestial bodies that interact in many ways, such as the moon's orbit around the Earth.