The chapter turns in its second half from a general principle to a specific person, and the way it names him fixes the audience of the book more precisely than most chapters do. How should the lord of ten thousand chariots take himself lightly in the eyes of the world?
Ten thousand chariots was a conventional measure of a major state during the Warring States period, and the phrase functions as a rank rather than a count. It is the ruler of a great power, not a village headman and not a private person seeking tranquillity. Chapters that use this register are asking what a head of state should do, and they assume the reader either is one or is advising one.
The closing lines are a threat rather than a suggestion. Take it lightly and the root is lost. Be restless and the ruler is lost. The word rendered ruler is the same one used in the opening line for stillness ruling restlessness, so the sentence is a pun that works in both directions: lose your composure and you lose the thing that governs you, and also, lose your composure and you lose your throne.
The chapter belongs with a substantial group in the book, including twenty-nine, sixty, sixty-one, sixty-five, seventy-two, seventy-four, seventy-five and eighty, that read as advice to a sovereign and make no sense addressed to anybody else. Readers who take the text as a manual of personal quietude have to do something with these chapters, and the usual move is to read the state as a figure for the self. That reading is old and productive, and it is a reading rather than the plain sense.