The moon rotates around the Earth in a kind of circle called an orbit. As we know, it is the sun’s light that illuminates the Moon. Now, the Sun always lights up only one side of the Moon. So, we would see only the part that is lit by sunlight. Since the Moon is always changing its position and is never at the same place, different sides are illuminated in different ways on different days. As we see only the part that is illuminated, the shapes appear to change every time the Moon changes its position. But it happens very slowly as the speed at which the Moon orbits around the Earth is not super fast!
It takes about 28 days for the Moon to go all the way around the Earth and return to where it started from.