Spectroscopy is the measurement of the response of a material as a function of frequency.
In its most basic form, it is how we see the world around us. Light from the sun, or a lightbulb, reflects from objects into our eyes. Different wavelengths of light are reflected differently by different objects, and our eyes process that reflected light to tell us the color and texture of the objects we look at. The measurement of this frequency dependence of the reflectivity, performed by our eyes and our brain, is an example of spectroscopy. Visible light is a small fraction of the electromagnetic spectrum, and its photons have energies in the range of 2 eV. This energy range works fine to give us some information about the world, but it is not perfect.