Temperature is proportional to the average translational KE of molecular motion, which is the motion that shakes and wiggles the molecule back and forth.
Work Step by Step
This is discussed at the bottom of page 286. Extra explanation for this sometimes-confusing term: You may be wondering what other kinds of KE there are, i.e., what KE is not counted as translational KE? If a molecule is modeled as a bunch of masses/atoms connected by springs/bonds, there is rotational KE due to overall spinning of the molecule. There is also vibrational KE due to the molecules shaking back and forth relative to each other. Finally, in the context of temperature, we do not count the overall motion of the object as translational KE. If a box is placed inside a high-speed jet plane, the extra overall KE doesn't contribute to a temperature rise.