Spectral lines refer to the discrete wavelengths of light emitted or absorbed by atoms or molecules when they undergo transitions between energy levels. These lines are characteristic of the chemical elements and can be used to identify the composition of celestial bodies, such as stars and galaxies. Spectral lines are important in various fields of research, including astronomy, chemistry, and physics, and have led to significant advancements in the understanding of the properties and behavior of matter in the universe.