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.