Emission lines refer to specific wavelengths of light that are emitted by atoms or molecules when they transition from a higher energy state to a lower energy state. These lines can be observed in various astronomical objects such as stars, galaxies, and nebulae, and can provide valuable information about the composition, temperature, density, and motion of these objects. Emission lines are commonly studied in the field of astrophysics to better understand the physical processes occurring in the universe and to probe the properties of different celestial bodies.