It's not exactly a symbol, Shmoopers, but this is the big one: you can basically sum up the whole of Les Misérables as a stand-off between justice and mercy. On one side, you have Bishop Myriel (and later Valjean), constantly ready to give people a second chance, to expect and hope for the best, and to treat people according to their individual situations rather than according to some abstract rule.
And then, on the other side, you have Inspector Javert and all the forces of law and justice—which, when you see how it's put into action, doesn't look nearly as just as it should. Justice means rigidity, harshness, and inflexibility. It means applying the same standards to poor innocent Fantine and the unredeemable Thénardier. And it means relentlessly pursuing a good man for a minor crime he committed decades ago.
In the end, of course, mercy wins, at least temporarily. So what's the lesson? Be excellent to each other. But we think there might be something bigger going on, too. It's hard to escape the fact that the absolute best guy in the book is a bishop, or that Valjean attributes his entire transformation to the work of God.
In fact, you could make a pretty good argument that the whole book is a narrative of Christian redemption. Mercy—i.e. New Testament values of forgiving prostitutes and tax collectors and what have you—is ultimately more powerful than Justice, i.e. Old Testament values of stoning prostitutes and sending plagues and so forth. In the end, Mercy destroys Justice, and we all live happily ever after. Except Valjean, of course, who has to sacrifice himself for his children to be happy, just like a certain New Testament figure we could name.
(PSA: You can take your finger off that "send" button, because this is Hugo talking, not us! We also know that saying "Hebrew Bible" is better than saying "Old Testament," but you can bet Hugo didn't.)