What are refractions?

Refraction is the bending of light as it passes from one transparent substance into another. This happens because light travels at different speeds in different materials. When light enters a substance at an angle (not directly along the normal), its speed changes according to the optical density of the material, and this causes the light to change direction. The bending is described by Snell's Law, which states that the ratio of the sine of the angle of incidence to the sine of the angle of refraction is constant, and is equal to the refractive index of the second medium relative to the first.

To put it simply, imagine you're pushing a shopping cart across a parking lot into a field of grass at an angle. The moment the wheels hit the grass, there is resistance, slowing down the wheels that pass over the grass first. This causes the cart to turn or bend its path; similarly, light bends when it changes speed moving between different materials.