When an RF signal meets the boundary between media of two different densities, it can also be refracted. Think of reflection as bouncing off a surface and refraction as being bent while passing through a surface.
A refracted signal will have a different angle from the original. The speed of the wave can also be affected as it passes through the different materials. A signal can be refracted when it passes through layers of air having different densities or through building walls with different densities, for example.