At the end of Chapter 51, what does Chuck symbolize?

Chuck serves as a symbol of innocence, he also represents the "normal" life that he and Thomas so aspired to finding upon their escape. With his death Thomas feels this dream die as well. There can be no home without the family. Thomas has lost a symbolic brother (and, it is strongly implied, possibly a biological brother as well), thereby fracturing the family dynamic that he, Teresa, and Chuck were beginning to build. Chuck's death is all the more bitter because it is both his observational skills that helps them in the Griever Hole as well as the fact that he sacrifices himself so that Thomas, the hero, may live.