What is Absorption band, fingerprint region?

An absorption band is a dip of the transmittance: the light of those wavenumbers is absorbed by a bond. Its position tells which bond, its width and its depth complete the picture. Below about $1500\,\mathrm{cm}^{-1}$, the spectrum is crowded with bands that depend on the whole molecule: this fingerprint region identifies a compound by comparison with a known spectrum, but is hard to read bond by bond.