A model you can talk about without mathematics. Which doesn't need mathematics to form an intuition about.
STR and GTR are perfect examples of models which you can help even a layman develop an intuitive understanding of by just talking to them, without mathematics.
You can certainly explain even something as sophisticated as quantum field theory to laymen if you take the right approach. Feynman did just this in his lectures on QED, now available in book form. He didn't use any math, even though he talked about his own solution to the entirely mathematical problem of renormalization in QED.
