The moon is basically a giant ball of rock 3500 km. across hanging in space. Its surface is actually pretty dark, with about the same reflectivity as a chalkboard or asphalt. However, it look bright to us because it’s sitting in full sunlight; the sun illuminates it, and it reflects that light down to us here on Earth. And because it’s a sphere, and orbiting the earth, the way we see it lit by the sun changes with time. That what causes its phases: