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, the standard video audio format is 48 kHz.
The Clock Source manages how the unit derives its digital clock. In a digital system, it's important each device that's sending signals digitally shares the same clock source, so that they can be synchronised.
If you're only using one device then you should select its internal clock source. If you're using multiple pieces of hardware connected together via digital means, the Clock Source on one unit must be set to Internal as the 'leader' and all other units in the chain should be set to the external connection as the "follower(s)".
For more information on which Clock Source to choose Click Here.