The Earth Orientation Parameters
The IERS Earth Orientation Parameters (EOP) describe the irregularities of the earth's rotation. Technically, they are the parameters which provide the rotation of the ITRS to the ICRS as a function of time.
Universal time. Universal time (UT1) is the time of the earth clock, which performs one revolution in about 24h. It is practically proportional to the sidereal time. The excess revolution time is called length of day (LOD).
Coordinates of the pole. x and y are the coordinates of the Celestial Ephemeris Pole (CEP) relative to the IRP, the IERS Reference Pole. The CEP differs from the instantaneous rotation axis by quasi-diurnal terms with amplitudes under 0.01" (see Seidelmann, 1982). The x-axis is in the direction of the ITRF zero-meridian; the y-axis is in the direction 90 degrees West longitude.
Celestial pole offsets. Celestial pole offsets are described in the IAU Precession and Nutation models. The observed differences with respect to the conventional celestial pole position defined by the models are monitored and reported by the IERS.
Seidelmann, P.K. 1982: Celest. Mech., 27, 79.