Work and mechanical energy I have come across the following lines in "Introduction to Mechanics" by Kleppner and Kolenkow.

A peculiar property of energy is that the value of mechanical energy $E$ is arbitrary; only changes in $E$ have physical significance. This comes about because the equation $$U_b - U_a = -\int_{a}^{b} \vec F \cdot \vec {dr}$$
  defines only the difference in potential energy between $a$ and $b$ and not the potential energy itself. We could add an arbitrary constant to $U_b$ and the same constant to $U_a$ and still satisfy the defining equation. However, since $E = K +U$, adding an arbitrary constant to $U$ increases $E$ by the same amount.

What does he mean by "$E$ is arbitrary; only changes in $E$ have physical significance."? What is this physical significance he is talking about?
The concept of potential energy is still unknown to me. The author has introduced this form of energy by mathematically restating the equation of work done by a conservative force. Potential energy will be discussed a little later. 
 A: In this case, the "physical significance" is referred to the evolution of the system: it will be defined only by the differences between certain values of $E$, then $E$ is arbitrary since whatever constant you add to $E$ (i.e. you change $E=K+U$ with $E'=K+U+c$) it will not be relevant since the constant will cancel when you evaluate the difference between two value of $E'$.
Think about a problem involving the gravitational potential energy $U=mgh$ of a body of mass $m$ suspended on an height $h$: whatever problem will you think about, the solution will ever depend on the difference between potentials evaluated on different height, and it will never depend on a single potential evaluated in a point. Then, if we redefine $U$ as $U'=mgh+c$  (where $c$ is an arbitrary constant) the physical situation of your problem will never change, since evaluating the difference between $U'$ in two different point, the constant $c$ will cancel and the result will be identical to the one you can obtain with the original potential.
In this sense, "no physical significance" means that the constant $c$ is not relevant at all in the evolution of your system. Being $E=K+U$, the reasoning done on $U$ can be extended on $E$, since whatever constant $c$ you add to $E$, it can be considered, for example, as a constant added to $U$.
