The French idiom à petit feu literally translates to "over a low flame". It's used to describe something that happens very slowly and gradually, often in a painful or torturous way, like something being slowly cooked.
In the song, Jul uses this powerful imagery to describe a relationship in crisis, singing "On se déchire à petit feu" (We're tearing each other apart slowly). This phrase perfectly captures the slow, painful burn of a love that is falling apart due to constant arguments.
French rapper Jul gives us a super honest look at a relationship that's going through a rough patch. He's not trying to be a perfect hero here. He admits he comes home late at night and that his girlfriend tells him, "Je sers à rien" (I'm useless). When she gets mad, his solution is to escape into video games: "Alors j'allume FIFA" (So I turn on FIFA). It's a real and sometimes funny look at how guys can mess up and avoid their problems.
But even with all the fighting, this is a love song at its core. The main message is right in the title: "C'est dur d'aimer" (It's hard to love). He knows they hurt each other and contradict themselves. Yet, in the very next breath, he tells her, "Tu sais que je t'aime à l'infini" (You know I love you infinitely) and wants to be with her his whole life. The song is all about that contradiction: love is messy and difficult, but he's not giving up on it.