The sampling frequency is the number of samples of a signal that are taken in a given time.
It indicates the accuracy with which a digital file describes the analogue sound that it represents. In general, the lower the frequency, the worse the sound quality; on the other hand, it will take up less disk space than a file produced at a higher sampling frequency.
CDs have a sampling frequency of 44.1 kHz, and DAT recorders of 48 kHz. MiniDisc players/recorders having a built-in sampling frequency converter can manage both frequencies.