Definition of Light
Light waves are a type of electromagnetic wave. Electromagnetic waves are different than waves on a string or sound waves in that they don't need anything to travel through. They're just waves of pure energy and because of this, they can travel through a lot of different materials, including empty space. They also move really fast, traveling through space at the speed of light (almost 300 million meters per second!). Although light waves can easily pass through materials like glass, water, and plastic, the waves slow down as they travel through these materials. When the light waves slow down as they pass from one material to another, that's when they're going through refraction.
When the light passes from air into a material like glass or water, it slows down and always bends toward a line drawn perpendicular to the surface of the material. This line is known as a normal line. When light passes from air into water or glass, it bends towards this normal line.
This explains why objects that you see underwater are never where they appear to be. Your eyes cannot tell that the light is not coming in a straight line, so you see an image of the object that is in a different place than the real thing.
Refraction of light
As you look at this image, notice that as light passes from air (on top) into water (on bottom) it bends down towards the normal line (shown by the dashed line).
You'll see that as light passes from water into air, it changes speeds, and this causes the light waves to bend. When you look at a pencil under the water, your eyes see an image of the pencil that's not where the pencil is actually located. Don't let your eyes fool you! The pencil only appears to bend at the point where it goes into the water because of this illusion.