I want to know what's the blind spot and why does everyone have it?

The blind spot in: Our visual field is due to the placement of the optic nerve. This is the part of the brain that enters the eye which contains primarily neurons (nerve tissue) and not photoreceptors (rods ; cones). The lack of photoreceptors prevent light from being absorbed for processing. We need the optic nerve to deliver the retinal signals to the brain for processing the images we see.