Before you can use this technique, you need to have an introduction to infrared spectroscopy and to what an IR spectrum is. Infrared spectroscopy deals with the interaction of infrared light with matter. The energy of an infrared photon can be calculated using the Planck energy relation. This shows that high energy photons have high frequency. You can see that both frequency and wavenumber are directly proportional to energy.
Molecules are flexible, moving collections of atoms.
The atoms in a molecule are constantly oscillating around average positions. Bond lengths and bond angles are continuously changing due to this vibration. All of the motions can be described in terms of two types of molecular vibrations. One type of vibration, a stretch, produces a change of bond length. A stretch is a rhythmic movement along the line between the atoms so that the interatomic distance is either increasing or decreasing.