Eclipses can only happen at New and Full Moon, when the Earth, Moon, and Sun are all in a straight line. But they don’t happen every New and Full Moon, because the Moon’s orbit is tilted by about 5 degrees. As the Earth and Moon travel around the Sun, the tilt of the Moon’s orbit changes direction relative to the Sun.
This is analogous to the way the tilt of the Earth causes seasons. Just like winter and summer happen every six months, eclipses tend to occur on a roughly six-month cycle.
Unlike most eclipse shadow diagrams, the first three animations here don’t greatly exaggerate the scale of the Earth and Moon. They are only 2x their true scale. The view is exactly perpendicular to the Earth-Sun line. The angle of the Moon’s orbital tilt and the “tapering” of the shadows are both accurate. The orbit happens to be calculated for the months preceding the April 15, 2014 total lunar eclipse.