Most English readers call Les Misérables by its original French name because it's not hard to figure out what it means in English – the Misérables. It can also be translated as "The Wretched," "The Poor," or "The Downtrodden." In other words, Victor Hugo wants to make sure we know that this book is about all the people who slip through the cracks of modern society. There's one passage in particular where Hugo refers directly to the book's title:
Certainly they appeared utterly depraved, corrupt, vile and odious; but it is rare for those who have sunk so low not to be degraded in the process, and there comes a point, moreover, where the unfortunate and the infamous are grouped together, merged in a single, fateful word. They are les miserable – the outcasts, the underdogs. And who is to blame? Is it not the most fallen who have most need of charity? (3.8.5.5)
In this passage, Hugo asks us to have sympathy for those who are not as well off as ourselves. Our lizard brains might want to blame them for their own terrible circumstances. Our heads might fill with thoughts like, "lazy bum" or "get a job, loser." But how much good does that do? A big fat none.
Compassion, on the other hand, and a helping hand at the right time—those can make a big difference in a person's life.