What causes lunar phases?

The phase of the moon (new, half, full, etc) is a function of how much of its lit surface we can see from the earth. Half the moon is always sunlit, half is dark, but as the moon orbits the earth, more or less of the lit face is pointed towards us here on earth.

The moon's synodic period around the earth is about 29.5 days, which coincides with the length of a month. Once in each of its synodic periods, its lit face is directly oriented towards earth -- a full moon.

Because the synodic period is less than some calendar months, on relatively rare occasions we can have two full moons within one month. The second of those full moons is referred to as a "blue moon" -- thus the phrase, once in a blue moon, which means "rarely."