The Sample Rate is the number of audio samples carried per second, measured in Hz or kHz (1 kHz being 1,000 Hz). For example, 44,100 samples per second can be written as either 44,100 Hz or 44.1 kHz. The higher the Sample Rate, the more samples of audio being carried by the device as well as a higher frequency range being captured, meaning the higher the Sample Rate, the higher the quality of audio being recorded. Standard CD audio format is 44.1 kHz, while to match the frame rate better, the standard Film Audio format is 48 kHz.
The Clock Source dictates how the unit derives its digital clock. In a digital system, it's important that each device that's sending signal digitally shares the same clock source, so that they can be synchronised.
If only one unit is being used, its Internal Clock Source should be selected. If multiple pieces of hardware are being connected together via digital means, the Clock Source on one unit must be set to Internal as the "master" while all other units in the chain should be set to the external connection as the "slave".
For more information on which Clock Source to choose Click Here.