Lunar Cycle is a very important cycle. Moon changes its shape and position everyday in its cycle. So our ancestor track these shapes and position to keep track of time.
Why our moon changes its shape or appearance? It's all about moon orbiting around Earth and the earth orbiting around Sun. Sometimes, while orbiting around earth moon comes in between sun and earth, we can see dark side of moon. We call it new moon.
As moon continues to orbit its, side is illuminated by sun. Now we can see crescent moon.
After orbiting some more distances we can see half of moon from earth. We call it quarter moon.
Finally as moon orbits further and is fully illuminated by Sun. We call it full moon.
A lunar cycle takes about 28 days to complete. Each day moon looks different in the sky. So they kept track how the moon appears in the sky. This help to keep track small amount of cycle like month.