Roughly speaking, folk theories are the theories that an ordinary viewer of Law & Order thinks are true. For daily life, many folk theories are close enough, but they often make nonsensical or wildly wrong claims when applied in any specialized context.

It appears that folk theories are slightly entangled with actual scientific theories, in that I doubt Victorian era common folk thought that the speed of light was a fundamental constant, and most moderately educated people now do have some concept of that idea.