Light rays usually travel in straight lines, but when they pass from one material to another they can be forced to bend (change direction and continue on a new straight path). The bending is called refraction. It happens because light travels at different speeds in different materials. If light rays travel through air and enter a more dense material, such as water, they slow down and bend into the more dense material. Light rays moving into a less dense material, such as from water to air, speed up and bend outwards.
This is a picture of seem and me doing reaction