Passantes is the feminine plural for "passers-by". It's a beautifully melancholic and poetic word that's not common in everyday conversation.
In the song, Joe Dassin imagines a life without his love, filled only with meaningless encounters. He sings of "des passantes endormies dans mes bras que je n'aimerais jamais" (female passers-by asleep in my arms whom I would never love), creating a vivid image of fleeting, empty moments. This word perfectly captures the song's theme of profound loneliness and the irreplaceability of a true soulmate.
This song asks a huge, dramatic question: What if the person you love most in the world just... didn't exist? Joe Dassin explores this idea with total devotion. He says his whole life would be pointless, singing, "Et si tu n'existais pas / Dis-moi pourquoi j'existerais" (And if you didn't exist / Tell me why I would exist). He imagines a world without this person as a gray place where he'd just be "pour traîner" (hanging around) with no hope.
He takes this idea to its most romantic extreme. Without this person, he claims he wouldn't be his true self, saying "Je pourrais faire semblant d'être moi / Mais je ne serais pas vrai" (I could pretend to be me / But I wouldn't be real). The most incredible part is when he says that if she didn't exist, he would have to figure out the very secret of life itself, "simplement pour te créer / Et pour te regarder" (simply to create you / And to look at you). It's a song about a love so complete that it defines reality itself.