The planets in our solar system orbit the Sun in very roughly the same plane. If you were to draw an ellipse on a piece of paper, the ellipse would be the planet's orbit and the surface of the paper would be the plane of its orbit. Now, as seen from the Earth, the orbital planes project onto our sky as a narrow band in which the planets move. Also, as seen from the Earth, the Sun moves within this band because it also contains a projection of the Earth's orbit around the Sun. It was so named because eclipses of the Sun occur within this band in the sky.