Sample Rate:

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. The standard CD audio format is 44.1 kHz, and the standard video audio format is 48 kHz.

Clock Source:

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 they can be synchronised. 

If you're only using one device, then you should select its internal clock source. If you have multiple hardware devices connected digitally, the Clock Source on one device must be Internal, the 'leader', and all other devices should be set to clock externally, the “follower(s)”.

For more information on which Clock Source to choose, Click Here.