Charbonner literally means "to charcoal" or "to blacken with charcoal". However, in French slang, it's a very common and evocative term that means "to work extremely hard" or "to hustle".
In the context of this song, Ninho uses it to describe the intense effort and grind required to make money and succeed, especially coming from a difficult background. It paints a vivid picture of relentless work to achieve one's goals.
Ninho's "Sourire" is a really honest look at the struggles of growing up and trying to make it, all while dealing with a lot of inner turmoil. He repeatedly tells his mom, "Maman, j'ai perdu le sourire" [Mom, I've lost my smile], which immediately sets a somber tone. He's constantly on edge, saying "Je suis tout le temps sur les nerfs" [I'm always on edge], and admits to smoking to forget his troubles, hoping his mom hasn't suffered enough.
He talks about a rough start to his day, "J'me suis levé du pied gauche, la journée a très mal débuté" [I woke up on the wrong foot, the day started very badly], and even mentions seeing a therapist after being judged. Despite the hardships, his love for his mother is a huge motivator. He vows to return and "couvrir d'or le cou de ma re-mé" [cover my mother's neck with gold], seeing her as a precious diamond. It's a powerful song about the sacrifices made for family and the search for happiness, even when it feels lost.