Trésor literally means "treasure" in French. It's a beautiful and evocative word that carries a sense of preciousness and value.
In this song, the artist uses "trésor" as a term of endearment, directly addressing a loved one. It highlights the deep affection and importance the person holds in their life, making it a central and very touching word in the lyrics.
"Trésor" is about needing to slow down without losing the person who matters most. The singer feels burnt out, using racing and flying images (a pit stop, a breakdown, getting high) to describe how worn down he is from pushing too hard.
He dreams of getting away, back to the sea at Ostend, and admits he can't listen to certain songs on the radio anymore because they remind him too much of what he's running from. But even as the world feels like it's ending, he keeps calling his partner "trésor" (treasure) and asking her not to push him away. His desires are messy right now, but her being there, steady and lovely, is what keeps him grounded.