What is refraction of light?

Refraction of light is the bending of light when it passes from rarer(less dense) medium to a denser medium or vice versa.

Explanation:  The bending of light happens as light travels at different speed in different medium for example light travels slower in water than compared to air as water is denser than air. That is why when you put a pen in a glass cup you will notice that the pen is slightly shifted (the part of the pen above water is travelling at a different speed than the part of the pen underwater).

Hope this you understand it