Emission line ratios refer to the relative intensities of different spectral emission lines emitted by a particular source, such as a star, galaxy, or nebula. By studying these ratios, astronomers can gain insight into the composition, temperature, density, and physical conditions of the emitting material. Different elements and ions produce characteristic emission lines at specific wavelengths, allowing researchers to determine the chemical makeup and properties of the source. Emission line ratios are commonly used in astrophysics to study the physical processes occurring in various astronomical objects and to understand the evolution and dynamics of the universe.