What are the "frustrated rotations" of molecules?

What are the frustrated rotation of molecules and how is this any different from the normal rotation of molecules?

This question originated from reading: A promising diagnostic method: Terahertz pulsed imaging and spectroscopy where it is stated:

"(…)frustrated
rotations and collective modes cause polar liquids (such
as water) to absorb at THz frequencies."

I am trying to understand how these rotations can cause such polarisations, but first I must understand the concept of frustrated rotation.