Some light hits an object and bounces back towards you. Your corneas do the bulk of the focussing. Your lenses fine-tune the focus so that the image hitting the retina is sharp. Your retina generates the colour and brightness information and does a little differential contrast enhancement and exposure compensation. This data heads off to the visual cortex in the occipital lobe (swapping the left and right nasal fields over at the optic chiasma (so that each visual field is dealt with in one chunk)). In the visual cortex are the equivalent of several layers of neurons all set to watch for different things; a series of layers which effectively recognise lines or edges at various angles, layers which recognise movement in various directions, further layers to collate the raw data and patch together the various different interpretations of the various aspects of the changing patterns of brightness, darkness and colour so that the image with which the consciousness judges the world is overlayered with a whole bunch of metadata describing the apparent size, shape, distance and motion of the contents of the visual field. You know where planes, lines, edges and points are, what is coming towards you, what is heading away from you and what everything is. I don't know whether or not the primate visual system is biased towards line-recognition because it did a lot of its more advanced evolution in trees but it's certainly comforting to look at them.