These two candlesticks are two of the only valuable things that Bishop Myriel has in his entire house, apart from his fancy set of silverware. We hear about these candlesticks early on, as the book says that visitors to Myriel's house "found nothing remarkable in it except two candlesticks of an antiquated design on the mantelpiece, which were presumably silver" (18.104.22.168).
In this instance, the candlesticks show just how modestly the Bishop is willing to live in order to give financial help to the poor people of his region. Myriel takes this generosity to new heights when he forgives Jean Valjean for stealing his silverware and gives him the candlesticks as a present, so long as Valjean promises to use the money to start a good, moral life.
By the end of the book, we might have forgotten about the Bishop's candlesticks, but Jean Valjean sure hasn't. In the final scene of the book, Jean Valjean passes away in the light of two candles that are mounted in these candlesticks: "He lay back with his head turned to the sky, and the light from the two candlesticks fell upon his face" (22.214.171.124). The appearance of the candlesticks here suggests that Jean Valjean has succeeded in keeping his promise to Bishop Myriel and has lived a good life.
Set It to Music
We're mostly avoiding the big elephant in the room a.k.a. the smash hit musical based on Les Mis. But there's a reason musicals are so popular: they really know how to get their point across. And in this case, we think we can't do much better than ol' Alain Boublil. In the tear-jerker opening number, Bishop Myriel gives Valjean the candlesticks, telling the ex-con that he has "bought your soul for God."
You can't get much clearer than that. The candlesticks are a symbol of Bishop Myriel's poverty and goodness, but they're also a symbol of Valjean's redemption. They're the literal price on his soul.