The ecliptic is the path the Sun seems to follow through the sky over a year, against the background of the stars. The Sun is not really sliding around us — the Earth is going around the Sun — but from the ground that is how it looks. That path lies on a flat plane, the same plane as Earth’s trip around the Sun. The Moon and the planets stay near this line too. It was named from eclipse: an eclipse can happen only when the Moon lies on this plane, so that the Moon can cover the Sun, or the Earth’s shadow can cover the Moon. The twelve star-signs of old sky-maps sit along the ecliptic. The word is also used as an adjective for things on that plane (the ecliptic plane), still the same idea.