In the ray traced image above several clear glass spheres refract light from varied positions within a checkerboard tiled room.
Refraction occurs when a light ray changes mediums.
Light traveling from air and going into water would be an example. The speed of the light ray changes upon changing mediums. In almost every case the direction of the light ray changes also. In the picture above light rays change direction as they go from air into the glass spheres, and they change direction again when they exit the glass spheres and enter air. This causes the checkerboard patterns to be distorted.