For largely historical reasons, our time-keeping system is reckoned by the position of the Sun. We can also choose to keep time by reckoning how long it takes for a specific star on the Celestial Sphere to get back to the same point in the sky night after night. Since one rotation of the Earth is all it takes to do this for distant stars, that is the Sidereal Day. NOTE Since one rotation of the Earth also moves the Earth $~1/365^{th}$ of the way around the Earth's orbit around the Sun, the Earth must rotate a little further to get the Sun back into the same position.

That $~1^\circ$ extra (did you catch why $~1^\circ$?) amounts to 4 minutes of extra rotation. It is on this 4 extra minutes of rotation that we base our 24h clock. So the Sidereal Day is 4 minutes shorted than the Solar Day (23h 56m).

Isn't that fun!?

The rate at which objects embedded in the Celestial Sphere move through our sky