The imaginary flat area that stretches out from the Sun's equator into space

The plane of our solar system. In the course of a year, the sun traces a path in the sky along the ecliptic. The Earth's axis is tilted at a 23.5° from the ecliptic (which causes the seasons).

(also called "the ecliptic" for short) The orbital plane of the Earth around the Sun. The line of the ecliptic on the celestial sphere is formed by the intersection of the plane of the ecliptic with that sphere. The reason the major planets and Moon appear in the sky close to the ecliptic is that the solar system is flat, and its orbital planes are very close to each other. We observe their motion (very nearly) edge-on.