We, as humans, have the gifts of our five senses. We use those senses to navigate our way through the world, learning and trying to make sense of things. Taking the quote literally, our eyes see something in our environment. Our brain processes that input into something that we can comprehend. What if our brain isn't able to comprehend the images that our eyes received? Perhaps our brain dismisses what 'doesn't make sense' and negates the input...as though we never even saw it.
Looking at the picture above, what do you see? I see a frog that is mid-jump heading for the water. I can also see his reflection in the water below him. Look even closer. Do you see a pair of frog eyes looking out at you, in front of and below the jumping frog? Pretty cool, huh?! The frog in the water is so well camouflaged that on a quick view, or with your attention focused on the action of the jumping frog, your mind dismissed this bit of information as irrelevant, and it probably didn't even register in your brain that there are two frogs in this image.
"People only see what they are prepared to see!"