The retina has two parts: the peripheral retina and the macula. If you imagine the retina as a circle with a bull's-eye at the center, the macula is like the bull's-eye - it is very small. The large area of the retina that surrounds the macula and makes up 95% of the retina is called the peripheral retina.
The peripheral retina gives us vision to the side called peripheral vision. It is this part of the retina that is at work when we see something out of the corner of the eye. Because the peripheral retina is not able to see detail clearly, we cannot use our peripheral vision to read, thread a needle, drive, or even recognize a face. If you see someone off to your side, "out of the corner of your eye," you may be able to tell who it is because you recognize the person's general shape, but you won't be able to see the expression on the person's face.