The sidereal day
is how long it takes the Earth
to rotate once on its axis.
Quick! How long does it take the Earth to rotate once on its axis?
24 hours? Right?
The actual answer is very close to 23 hours 56 minutes.
The reason is that the Earth moves a little in its orbit every day, so the Sun moves around in our sky due to this, relative to the stars- it makes a rotation every year- and means that our (solar) day is over 1/365.25 longer than it should have been per day just due to the rotation of the earth. Hence the Day is 24 hours, but the rotation of the earth takes almost 4 minutes less.
(It's not exactly 4 minutes, because the Babylonians chose 360 for their numbering, and we inherited this for our time system, but 360 is very close to 365- 1/360 of 24 hours turns out to be 4 minutes exactly.)