What is a source of law?

What is a source of law and why is it important to religion? This is vital to understanding how to correctly apply Dr. Khalifa's teachings in relation to the Quran itself. This is something he emphasized, at least, 25 times right up until his death. See here. https://rumble.com/v461s8c-what-is-submission-a-comparison-of-two-paradigms.html