The tradition teaches that a human being has seven bodies, each finer than the one before. There is the physical body, and the vital body that keeps it alive. Then come the astral body of the feelings and the night, and the mental body. Beyond them are the body of the will, and two that belong to the spirit.
You can feel the second one now. The vital body is the warmth and the faint hum of life under the skin. Without it the physical body could not live for a moment.
On page 46 you met a further teaching. Most people have only loose, borrowed versions of the astral and mental bodies, lent by nature. The real ones, the books say, are built, slowly, by the kept energy of Part III. So is the body of the will.
This is why the energy matters so much. It is not only for calm days. It is the material from which the finer bodies are made.
You do not need to see any of this to work with it. You need only notice the life in your own body, and stop spending it carelessly.