Each sattelitesatellite is sending exact (atomic) time, and gpsGPS receiver calcualtescalculates thanks to it, it's position. There is also some additional data, like position of sattelitesatellite (almanac) which is also required to perform calcualtionscalculations, and a few minor things.
In order to increase accuracy, data may be sent on two diffrentdifferent frequencies (L1 which is available for everybody, and L2 which is restricted for US military).