UT1 describes the non-uniform daily rotation of the Earth. At any particular time, the rotation angle of the Earth differs from what would be predicted if the length of day were exactly 24 hours. So, if you were to measure the rotation angle at exactly 12:00 P.M. for many days in a row, you would find that your measurements would vary from day to day, with the overall trend showing that the rotation angle gradually decreases over time. After a long period of time, you would have to add one second to the day, called a "leap second", to compensate for this.
