It is basically the bending of light rays when they travel through different media.
The refraction of light or the bending of light rays occur when they pass through different media (ex: a laser beam passing through air and then through a water bottle) with different refractive indexes.
In simple terms, the bending of light or the refraction of light occurs when the rate at which light travels (velocity of light) through different media changes.
Also, the optical density of the travelling media determines the magnitude of the light refraction.
So here are the basic things about refraction of light.
the light refraction depends on the refractive index which is given by the refractive index of the medium
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the speed of the light in vacuum divided by the speed of light in the medium (the ratio of velocity of light in a vacuum to the velocity of light in the specified medium)
the medium in which light travels slower and the refracted rays bend towards the normal when they traverse through is called the denser medium/slower medium
the medium in which light rays travel faster and the refracted rays bend away from the normal when thy traverse through is called the rarer medium/fast medium