Since the light travels in the same medium its speed does not change but in the case of refraction, there is a change of medium and hence such speed changes. Also if the wave is an electromagnetic wave then its speed will not change as it travels with a constant speed.

What does refraction not change?

When a ray of light passes from one medium to the other medium, the physical quantities related with this ray of light such as velocity, wavelength, amplitude, etc changes such that the frequency of the wave always remains the same. Therefore, the frequency of the light does not change on its refraction.

When light is refracted there is a change in its?

The frequency is the number of waves passing a fixed point in a given period of time. The frequency of the wave does not change. The frequency is only determined by the source. When light is refracted, there is a change in its wavelength and speed.