The whole celestial sphere seems to rotate about an axis through its center. We see this axis as two centers of rotation (and hence fixed points) in the sky: the north celestial pole (NCP) and the south celestial pole (SCP).
(The south celestial pole is diametrically opposite the NCP, and hence visible only in the southern hemisphere of the Earth.) Of course really the sky is not rotating; rather the Earth is rotating. The apparent rotation centers in the sky are just the projection of the Earth’s rotation axis into the sky.
Exactly between these two centers of rotation we sketch the imaginary line of the celestial sphere’s equator. The celestial equator is just the projection of the Earth’s equator into the sky.