There are, of course, many symbols in Hugo's novels, but two of the most important are the Notre Dame Cathedral and Esmeralda's red baby shoes.
The cathedral serves as a symbol of France, specifically, it represents the possibility of a return to France's former glory.
Esmeralda's red shoes are a symbol of innocence and purity. Recall that Esmeralda was kidnapped by gypsies when she was a child. Her mother managed to save one of her baby's red shoes. Years later, young Esmeralda meets with a gypsy who has the mate to her baby shoe. The gypsy tells Esmeralda that as long as she remains virtuous, that one day, she and her mother will be reunited.