The celestial equator is a great circle around the celestial sphere in the same plane as the Earth's equator. Declination is measured from the celestial equator. Similarly, the celestial poles are the points on the celestial sphere directly above the Earth's north and south poles.
In contrast, the ecliptic is in the same plane as the Earth's orbit around the Sun. The angle between the two, 23.4° is the familiar angle of the Earth's axis.