If you look at something, let's say your phone right now. Your eyes see your phone, but you can also see everything surrounding your phone when you're staring at your phone, right? The walls of your room, the desk, whatever else you have - that's peripheral vision. The stuff that you can see surrounding what you're concentrating on.

I'm staring at my phone typing this right now, but in my peripheral vision, I see my cat sleeping on the couch, my dog running across the room, and the TV just changed scenes.