The Celestial Sphere is a way of imagining the sky as a giant imaginary ball, with Earth inside it, in the center of the ball, that has all of the stars painted, like little white dots projected on the inside of a planetarium’s domed ceiling.
When we stand on the earth and look up, we see the inside of the giant ball, like a dome above us. However, we can only ever see half of the celestial sphere at once, because the other half is blocked by the Earth itself, and is below the horizon.
The Celestial Sphere uses a coordinate system based on Right Ascension and Declination. Right Ascension is like longitude, indicating how far around the sphere’s “equator” a star is, and Declination indicates how far up from the sphere’s equator a star is. The North Pole is at the top of the sphere, “declined” 90 degrees up from the “equator” of the celestial sphere. A star on the Celestial Sphere’s “equator” would have a declination of 0 degrees.
Just as the Earth is a sphere, but can be drawn as a rectangle, the same can be done with the Celestial Sphere: