Like terrestrial coordinates (ie longitude and latitude), two coordinates define a point on the celestial sphere. The rotation of the earth defines a direction in the universe and it is convenient to base a coordinate off that rotation/direction. The celestial equator is the line coplanar with the earth's equator (and 90° to the axis of rotation). The north celestial pole is directly above the earth's north pole and likewise for the south celestial pole. The coordinate indicating where an object is between those poles is declination.