Spectral lines are distinct lines that appear in a spectrum when light is emitted or absorbed by atoms or molecules. Each element has a unique set of spectral lines, which correspond to specific wavelengths of light. These lines can be observed in both emission spectra, where light is emitted, and absorption spectra, where light is absorbed by a substance. The study of spectral lines is crucial in fields like astronomy and chemistry, as they help identify the composition of stars and other celestial bodies. By analyzing these lines, scientists can determine the presence of elements such as hydrogen, helium, and carbon in distant objects.