Light really does travel slower in transparent or translucent materials such as air, glass or water than it does in a vacuum. If it didn't, lenses would not be possible.
When passing through such a medium, light will have a slower speed than in a vacuum; the ratio of c to the observed phase velocity is called the refractive index of the medium.
The refractive index of a medium is a measure for how much the speed of light is reduced inside the medium. For example, typical glass has a refractive index of 1.5, which means that light travels at 1 / 1.5 = 0.67 times the speed in a vacuum.
