At the back of your eye is the retina. Your retina is made up of light-sensitive cells which send messages to your brain about what you see. Everyone has a spot in their retina where the optic nerve connects. In this area there are no light-sensitive cells so this part of your retina can’t see. We call this the blind spot.
Most of the time you don’t notice your blind spot because the spot in one eye doesn’t match the spot in the other eye. Each eye supplies information to the brain, filling in what’s missing. Also, sometimes the brain will fill in the missing information with what it thinks should be there. That causes one kind of optical illusion.