Binary Opposition is the categorising of opposites. E.g Light Vs Dark, good Vs evil. When we apply this to character creation, having a character who is wholly good or wholly evil isn't very exciting. This is because being somewhere in the middle gives characters a lot more depth and believability. As humans it is hard to believe that one person could not have a scrap of good in them or be 100% nice.
Take the Gollum/Smeagol idea from Lord of the Rings. First he was nice, then he saw the ring, went a bit evil, lived in a cage for ages, found Frodo, was nice, was evil, plotted, fought with himself etc etc. All this makes for much better storyline than if Gollum had just been an evil creature who Frodo captured and forced to lead him to Mordor.