Abdallah gave a good description. I’m just going to add that you can extract some important parameters from the propagation constant (modal index). For example, the imaginary component of a complex propagation constant will give you information about the loss of the mode. The loss can change from mode to mode. The different modal indices also results in intermodal dispersion, which is one of the main reasons most systems use single mode fibers. It is also important information for when you are studying coupling of modes, in a butt coupled situation it will tell you how much power is reflected.