Câlina is a very endearing and affectionate term, similar to "my darling" or "my sweetheart" in English. It's derived from the French word "câlin," which means a hug or a cuddle.
In the song, Kendji Girac uses "Ma Câlina" as a term of endearment for the person he loves, emphasizing a deep sense of affection and intimacy. It's a unique and tender way to address someone, making it stand out from more common terms of endearment.
"Ma câlina" is Kendji Girac talking to someone who has already left. The title is his own coinage: câlin is a cuddle, so ma câlina is roughly "my cuddly one" — a pet name that existed only inside this relationship, which is why the chorus can repeat it for two minutes without it wearing out. The opening gives the before-picture in one image: "Sans toi / Mon cœur était un nomade" [Without you / my heart was a nomad]. She stopped the wandering, and her leaving starts it again.
What he regrets is specific, and it is silence. "Mon cœur ne te parlait pas" [My heart didn't speak to you] is the admission the chorus is built on, and the verse says the same thing from the other side: "Je n'parle pas d'amour / Mais j'y pense fort tous les jours" [I don't talk about love / but I think about it hard every day]. He felt it, he never said it, and the hook does the arithmetic — "J'aurais tout fait pour toi" [I would have done anything for you] — in the conditional past, the tense French keeps for things that did not happen. By the final verse he is reduced to asking the empty room "Ma câlina tu fais quoi?" [My darling, what are you doing?], which is what is left over when the talking never got done.