Candlesticks usually contain candles, which emit light. Light is often associated with spiritual awakening or understanding. For example, in Genesis, God creates light, separates the "light from the darkness," and acknowledges that light is good, or goodness (1:3-4). Later, the psalmist refers to God's commands as a "light unto [his] path" (Psalm 119:105). Later still, John the gospel writer acknowledges Jesus Christ referring to himself as the "Light of the World" (8:12). All of which shows us that light, such as emitted from a candle, symbolizes spirituality and spiritual awakening. The candlesticks and light they are associated with would especially symbolize spiritual awakening when associated with the Bishop of Digne.
After Jean Valjean is released from prison, the only one who is willing to take him and treat him like a human being is the Bishop of Digne. Sadly, still caught up in his old ways of suffering and committing evil just to survive, Valjean steals the silver the Bishop used during the meal. The silver represents the Bishop's goal of treating Valjean with respect; hence, later when a policeman catches Valjean with a lot of silver in his knapsack and returns him to the Bishop, the Bishop responds by saying that he had given Valjean the silver as a gift and tells him to take the candlesticks as well, reminding him that they are worth about two hundred francs. He also tells Valjean to always remember that he "promised to use this money in becoming an honest man" (Book 2, Ch. 12). Not only that, the Bishop speaks of redemption and saving grace when he says:
Jean Valjean, my brother, you no longer belong to evil, but to good. It is your soul that I buy from you; I withdraw it from black thoughts and the spirit of perdition, and I give it to God. (Book 2, Ch. 12)
Ironically, Valjean carried his own candlestick before he met the Bishop. It's described as large, black, and iron, and a "miner's candlestick" (Bk. 2, Ch. 12). Convicts carried the candlesticks when they did mining work in the hills of Toulon. We see Valjean removing the candlestick from his knapsack before he steals the silver, as if he plans to kill the Bishop with it. Hence, we see that the black iron candlestick symbolizes Valjean's soul, made black from mistreatment and suffering, while the silver candlestick symbolizes his newly redeemed soul, redeemed through compassion, mercy, and brotherly love and respect. Plus the light the candlesticks emit symbolize spirituality. Both the black candlestick and the silver candlestick would equally symbolize the soul in its present spiritual state because even a tormented soul has access to light, albeit dim light, meaning that even a tormented soul can understand spirituality and receive spiritual redemption.