Eclipses only occur if the satellite of a planet is located within 0.5 degrees of the plane of the Ecliptic, on a line which passes through the center of the Sun and the Earth. The Moon travels along an orbit which is inclined by 5 degrees to the Ecliptic plane, so there are only two opportunities each month when it passes through the plane of the Ecliptic...called the ascending and decending nodes. These two points connected to the barycenter of the Earth-Moon system ( roughly the center of the Earth ) define a 'line of nodes', and eclipses of the Sun and Moon will occur if this line of nodes coincides with the line drawn between the center of the Earth and Sun. Again, the Moon also has to be within 0.5 degrees of one or the other of the nodes so that the disk of the Sun is partially or totally covered in a solar eclipse. A similar argument explains why lunar eclipses do not happen every full moon at the node opposite the Sun from the Earth.