How does a red shift indicate that the universe is expanding?

nice question! actually when a wave approaches an observer the wave length of the wave decrease and as it move far from us its wavelength increase. it is quiet difficult to understand so use your imagination. as the wavelenght increase the spectrum will be shifted toward red spectrum and thus called red shift. when Edward Hubble studies spectrum of different star he noticed that because of some color spectrum were missing, other spectrum got shifted to ward red. it it moved toward red then that mean that wavelength was increasing and if wavelength increases then that means light moves away from us and thus he concluded that each and every star and galaxy was moving away from each other