The note refers first to the irony of Jean Valjean's life. He lived through countless struggles. He was arrested for stealing a loaf of bread to feed his seven starving nephews and nieces and was sentenced to prison for 19 years. While in prison, he was treated like an animal and tormented by the fact that he had been taken away from caring for his sister and her starving children. He is so tormented that it eventually hardens him, and he is described as having had "entered the galleys sobbing and shuddering; he emerged impassive. He had entered in despair; he emerged gloomy" (Vol. 1, Bk. 2, Ch. 6). Hugo's point is to show the injustice of the law and how it robs a man's soul. However, later, Valjean's soul is redeemed by the Bishop of Digne who treats him respectfully as a man and a brother of faith and gives Valjean everything the Bishop owns of value to use the money to redeem himself.
Valjean begins a new life as a just and moral mayor under the name M. Madeleine but is later recognized by Inspector Javert and must flee in order to rescue Cosette, Fantine's daughter, whom he promised he would always protect. In fact, several times in the book he must flee from the law, with no other motive than fulfilling his promise to protect Cosette. At one point he even frees Inspector Javert from being executed; Javert then commits suicide, freeing Valjean from his enemy of so many years. But even though the Bishop of Digne redeemed Valjean's soul so many years ago, and now he is free of his enemy, Valjean remains haunted by his past. When Cosette marries Marius, Valjean confesses to Marius about who he really is. Gradually Marius forbids Valjean to continue seeing Cosette. Being torn from Cosette causes Valjean so much suffering that it kills him. Therefore, the irony of Valjean's life is that sacrificing himself for others by stealing a loaf of bread for seven children leads him to a life of suffering. His life is further ironic because, even though the Bishop of Digne helped Valjean redeem his soul, Valjean still lives tormented by his past, until he finally dies of suffering.
The line in the note stating, "He died when he had no longer his angel," refers to the fact that Valjean gradually decided to stop coming to see Cosette and Marius gradually forbade him to come. Again, the irony is that Marius's decision was based off of his belief that Valjean was not trustworthy because that's how Valjean spoke of himself when he made his confession to Marius. Valjean intentionally left out of the story that he had been the noble mayor M. Madeliene or that he had saved Marius's life, carrying him home underground through the sewers. Valjean had even left out the fact that he had spared his enemy's life, Inspector Javert. The truth is that Valjean was a redeemed man, or as Marius phrases it, he is "an angel." Marius is even shocked that Valjean has asked for forgiveness when all he has done is save Marius's life and repeatedly sacrifice himself. But Valjean is unable to see himself as fully redeemed until he has received forgiveness from his angel, Cosette.
The note on Valjean's gravestone reminds us of Hugo's central theme in Les Misérables, that oppression brings suffering and that redemption of the oppressed is essential.