The ecliptic is the region of sky (region of the celestial sphere) through which the Sun appears to move over the course of a year. This apparent motion is caused by the Earth’s orbit around the Sun, so the ecliptic corresponds to the projection of the Earth’s orbital plane on the celestial sphere. For this reason, the Earth’s orbital plane is sometimes called the plane of the ecliptic.

Due to the tilt of the Earth’s rotation axis with respect to its orbital plane, there is an angle of 23.5^o between the ecliptic and the celestial equator.

As the Sun moves along the ecliptic during the year, it appears to pass through 13 constellations (called the zodiacal constellations).
The name ecliptic comes from the fact that eclipses only happen when the Moon passes through this plane (as only then are the Sun, Moon and Earth aligned).