Explain it from the meaning alone: the introduction, the person who sends a message between the subject and the object. In ancient times, two princes met, and an attendant passed messages on both sides, similar to a modern portable translator. So it is extended to: intermediary, introduction, media and so on.
Quality: matter. Act as an intermediary for its delivery function. For example, what others say "reaches" your ears, and the substance that plays an intermediary role is air; The stove fire "conducts" the heat energy in the pot, and the intermediate substance is metal; In power transmission, the intermediate substance is a conductor.