What is Sharpness, Basic Sharpening Theory, the Starting Point for Learning to Sharpen Tools

Today's video is about basic sharpening theory, looked at through the question, what is sharpness?  This information is where learning to sharpen should ideally start.  Information like which type of sharpening stones to use, techniques for using them, which tools are sharpened at what angle and so on, are not much use without understanding what sharpness is and the factors that create it.  This information illuminates the goal and by extension possible ways to go wrong in pursuing it.