Phases of the moon is caused by the moon going around earth. This is also the reason the phases of the moon are related to 1 month.

To be more accurate, the phases of the moon is caused by the relative position of the moon with respect to the sun to an observer on Earth.

Sun, Moon, and Earth forms three sides of a triangle on any given day. Because it takes a whole month for the moon to move around earth its movement in one day is not significant on earth.

So to an observer on Earth the moon looks stable on any given night. Because of the "triangle" that moon forms with Sun you will see no moon when moon comes towards the middle of Sun and Earth. You will see a full moon when the Moon goes to the other side of the Earth away from the sun.