This is a change in the direction a wave travels when it enters a new substance. The wave could be a light wave, a sound wave or any other kind of wave. For example, when a light wave passes from air into a triangle of glass, called a prism, the higher density of the glass causes the light wave to slow and bend. The refraction separates the various wavelengths of light. The separation causes them to leave the glass triangle at slightly different angles. Because each wavelength corresponds to a different color to our eyes, the glass separates a white light into a rainbow of colors.
In a sentence
If a board of wood and plexiglass is thin enough, it may allow light through without refraction — making it appear clear and transparent.