The earth rotates on its axis and one full rotation takes 24 hours to complete. We’d be very surprised if you didn’t already know that, but what you may not know is that the speed of the earth’s rotation can change from day to day and year to year.
So a true solar day is not exactly 24 hours, but the variations are a matter of seconds. A mean solar day is based on the yearlong average, but the basic concept of a solar day itself is the length of time for the earth to complete one full rotation on its axis.