The moon's orbit is not perfectly aligned with the equator and the earth rotates on an axis around 20 degrees off, so the earth and the moon only line up with the sun once or twice a year.
A lunar eclipse occurs when the Moon enters the Earth's shadow. A solar eclipse occurs when the Moon's shadow falls on the Earth. They do not happen every month because the Earth's orbit around the sun is not in the same plane as the Moon's orbit around the Earth.
