Térébenthine is the French word for "turpentine", a strong-smelling solvent derived from pine resin, often used to thin paint. It's a very unusual and powerful word to find in a pop song.
Louane uses it as a striking metaphor for a hidden, intense feeling in her chest. Like turpentine, this emotion is potent, volatile, and perhaps even a bit dangerous, suggesting a love that is all-consuming and unforgettable.
This song is a countdown of a love affair, starting with the most important day: "Jour un" [Day one]. Louane describes this first day as "l'amour suprême" [the supreme love], a moment so perfect she immediately wants a "jour numéro deux" [day number two]. The relationship progresses quickly, from a "suite à l'hôtel" [hotel suite] on day two to life feeling like "velours" [velvet] by day nine. She admits her feelings are a "dépendance à l'amour" [dependence on love], and there's no point pretending otherwise.
The whole song circles back to that first day. The chorus explains that "Jour un" is "celui qu'on retient" [the one we remember]. It's a powerful memory that can be erased if her lover replaces her, but it's also the feeling that "revient" [comes back] when they are close. It's the beginning you always return to, the foundation of the entire relationship, for better or for worse.