IR Spectroscopy measures the vibrations of atoms, and based on this it is possible to determine the functional groups.5 Generally, stronger bonds and light atoms will vibrate at a high stretching frequency (wave number).
More specifically, IR measures the absorption of infrared energy as infrared light is absorbed by the bonds. Some of the bonds stretch, scissor, wobble symmetrically or unsymmetrically, and so forth. IR is essentially telling you what functional groups are present because bonds in various functional groups absorb light at different "wavenumbers." It is a great tool for identifying functional groups but gives little information about structures, except for how a benzene ring is substituted.