Refraction is the term used to describe the change in direction of a light wave due to a change in its transmission medium.
In other words, when light comes in contact with certain surfaces, like water or glass, the light will get slightly bent because those surfaces affect the speed at which light travels through them.
The best example of Refraction can be seen by placing part of a pencil in water.
The part of the pencil that is placed in the water will appear bent where the pencil and water meet due to Refraction.
Here is a very simple example showing how Refraction works.
The part of the pencil that is under the water will appear bent when viewed from the camera due to Refraction.
This happens because the light waves from X, the original pencil position, are being affected by the waters refraction and appear to actually be coming from Y when viewed from the camera.
This is happening because the speed at which light travels changes as it hits the water and then changes again as it hits the air.