It simply means we overwork our physical bodies, either through repeated activity, lack of rest, or too much mental stimulation. The symptoms show up as physical problems that won't go away, and depression, lack of energy, and even death. Let me show you. Mark is leaning forward with great interest. On the screen the fraying cable becomes magnified and rocks back and forth over a pulley.
All mechanical things wear and break from use.
Our muscles and nerves are no different. Overuse without allowing the body time to repair, causes muscles to thin or to tunnel. Mental stress causes muscles to stay tight, or spasmed, causing the same damage. Even nerve fibers can fray from too much flexing in one position. They can become raw and irritated, or actually break strands.