Otherwise affectionately known as the LGN or "Lateral Geniculate Nuclei" are burdened with the role of relay station for the eyeballs.
Essentially, they recieve input from the retinas via the optic nerves, modulate this information in a fashion that is both subtle and not entirely clear.
The LGN then spits this information via specific neural pathways to an area at the posterior most area of the brain (or the occipital pole) aka the primary visual cortex where your brain interprets these electrical signals into what you can "see".
As an aside, one of the most interesting aspects to this visual pathway is its entirely faithful retinotopic map. That is, information that is recieved by the eyeballs is eventually spat out to the primary visual cortex 1 for 1. No summation, no concatenation, no mixing of signals. In other words, if you had a super-duper-really-sharp scalpalTM and sliced out a single line of rods and cones from your retina, that "black line" of dead/dysfunctional nerve axons will be maintained all the way down the cabling right through to the back of your head.