Did you know that you have a blind spot? I'll prove it to you.
Close your left eye and look at the plus sign above. Then slowly move slowly towards and away from your screen. At some point, when you are about 12 inches away (depending on the size of your screen that you're using), the black dot disappears. Your literally can't see a very small section of vision! Your brain just "fills in" the space with its surroundings- which in this case is the white background.
The blind spot is there because inside your eyeball, where the optic nerve sits, there are no photoreceptor cells (ie, rods and cones).
The retina is everywhere you see the orange color inside the eye in the photo above. The retina is neuro-sensory which means its part of the nervous system.
The optic nerve is where all of the neuro-sensory information from the retina gets collected and passed to the brain. The optic nerve begins in the back of the eye where you see the yellow circle.
The retina exists just about everywhere in the back of the eye, except for where the optic nerve is. So anytime an image gets focused on the back of the eye, that small piece of the picture that lands on your optic nerve is lost. Luckily, when we have both eyes open, the blind spot doesn't matter. Because what one eye DOESN'T see, the other eye DOES see! Our brain puts the images from both eyes together!