The word 'Seeing' (in it's most usual context), can be pretty much defined as meaning the perception of light.
We have a very good understanding of how light is physically detected by eyes and how eyes generate nerve signals representing the light detected.
As for how the brain is able to translate the nerve signals as recognisable objects, as far as I know this remains largely undiscovered, other than that we do know that certain areas of the physical brain are involved.