Refraction is the change of direction of light as it passes from
one transparent substance into another.
Refraction takes place because waves change speed when they cross a
boundary. The change in speed of the waves causes a change in speed of
the waves causes a change in direction, unless the waves are travelling
along a normal.
A light ray will refract when it crosses from air into glass. It is
refracted towards the normal.
The refractive index of substance is a measure of how much the substance can refract a light ray.