Molecules are not static. Vibration of bonds occurs in the liquid, solids and gas phase. These vibrations are important in many areas for analysis of functional groups, structural identity, and fingerprinting. The motion in a normal vibration can be approximately described as a kind of simple harmonic motion. In this approximation, the vibrational energy is a quadratic function with respect to the atomic displacements.
In CPMD only the corresponding frequencies are computed, neither their intensities (at least not directly) nor whether a specific mode is active in infrared- or Raman spectra is determined. There are several options to calculate vibrational spectra with slightly different properties.