You've just asked for the entire working premise of Chinese Medicine to be explained both in a couple sentences and "asap" - it's simply not that easy. The best explanation for laypeople would be to deeply understand what we are saying in "What Does Acupuncture Treat? - treating the cause and not the symptoms". To discuss more directly about choosing acupuncture points - the article "Acupuncture Point Treatment Plan" will get you in to the laypersons ballbark so to speak.
Long story short yin, yang, the five elements, etc. are just concepts to understand the systemic working order of the body. Trying to simplify them down to use this point for yin deficiency - is nearly as wrong as use this point for a cold - it's just not that simple for better or worse. One of the primary values of Chinese Medicine is the ability to tailor treatments to each individual and that is where good results come from.