Refraction is what occurs when a light (such as an electromagnetic light wave) enters a new medium (type of matter- such as solid, liquid, or gas) that causes its speed to change. For example, when light passes from a fast medium (gas) to a slower medium (such as a liquid), then the light ray will bend toward the normal to the boundary between the two medium. 

Refraction is what accounts for the "bending of a straw" when it is placed in a cup of water. It is also what causes a spear-fisherman to have difficulty in pinpointing the exact location of the fish that he/she intends to spear. Finally, a prism works based on the principles of refraction. Long wavelengths (red) refract less than short (blue or violet), so the colors of visible light separate.