Rouiller literally means "to rust". It's a fantastic example of a word used metaphorically to create a powerful image.
In the song, OrelSan says, "Si j'suis parti, c'est parce que j'avais peur de rouiller" (If I left, it's because I was afraid of rusting). He's not talking about metal, but about himself—feeling stuck, stagnant, and slowly decaying in his rainy, unchanging hometown. It perfectly captures the feeling of needing to escape to avoid losing your shine.
Ever felt a bit stuck in your hometown, but still loved it anyway? That's the heart of "La Pluie." OrelSan paints a super specific portrait of his home in Normandy, France, a place he says has "Toujours autant d'pluie chez moi" (Always so much rain where I'm from). He describes a normal, maybe even boring, life with funny details like coming from "la classe moyenne, moyennement classe" (the middle class, moderately classy) and listening to French pop star Julien Clerc in the family minivan.
But the song's main idea is finding the good in the bad. Despite the constant rain, which feels like a metaphor for all the little frustrations of his town, he insists, "Mais il fait quand même beau" (But it's still beautiful anyway). He remembers being a kid, stuck inside, just wanting the rain to stop so he could go skate. Now that he's left, he admits something surprising: "Trempé, j'aurais jamais pensé / Qu'le mauvais temps finirait par me manquer" (Soaked, I would have never thought / That I would end up missing the bad weather). It's a song about how the things we complain about are often the things that make us who we are, and that we can miss them when they're gone.