When we observe a star directly without any obstruction between us and the star, we see light of all colors, so the spectrum of the star looks continuous, without any gap. If we observe a diffuse cloud of gas directly, we only see light of specific colors, so the spectrum of the gas looks discrete, full of gaps. And if we see the light coming from a star through a cloud of gas, some colors are absorbed by the cloud leading to a continuous spectrum with some gaps. The gaps show the missing colors that have been absorbed by the atoms and molecules of the gas. In physics and chemistry, we call them absorption lines because they appear as dark lines in a spectrum. In the brightness-wavelength plot, we see a sudden drop in brightness at specific wavelengths corresponding to the missing colors.

The absorption lines arise because the electrons inside the atoms and molecules jump from lower a energy-level to a high-energy level or sometimes completely escape from an atom by absorbing the light of a specific color, frequency or wavelength. The opposite also happens. An electron inside an atom can jump from a higher-energy level to a lower-energy level by emitting or radiating light of a specific color. The spectrum of a cloud of gas is an emission spectrum, full of emission lines. The light from a cloud comes only because of some specific electrons that are going to some specific lower levels emitting light of some specific colors. Note that the absorption spectrum is just an inverted emission spectrum.