Spectroscopy is a method of measuring light split into it's spectrum, as by a prism. The atoms of every element emit and absorb specific wavelengths of light which appear as a specific pattern of bands or gaps in it's light spectrum. As we attempt to learn about our vast universe while confined to our tiny vantage point of earth, spectroscopy is how we know what distant planets, stars, and galaxies are made of, their mass, temperature, and more, without needing to travel to them. It's even how we know that our universe is expanding. Very distant galaxies don't just happen to emit more red light, the pattern of their emissions spectrum is red shifted and thus racing away from us in all directions. There is a beautiful wealth of knowledge hidden in the light!