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.
Joe Dassin asks one dramatic hypothetical question and refuses to let it go: "Et si tu n'existais pas, dis-moi pourquoi j'existerais?" ("And if you didn't exist, tell me why I would exist?"). It sounds sweet at first glance, but he pushes the idea so far that it turns completely over-the-top. Without this person, he claims he would just wander around purposeless ("pour traîner dans un monde sans toi" / "to loiter in a world without you"). The French verb traîner is great for English learners: while it literally means to drag, here it describes drifting around like an aimless slacker with nowhere to go. Without her, he says he would only be "un point de plus dans ce monde qui vient et qui va" ("just one more dot in this world that comes and goes"), faking his entire personality with no real core.
His backup plan is where things get gloriously theatrical. If she were not real, he insists he would sit down and invent love from scratch, "comme un peintre qui voit sous ses doigts naître les couleurs du jour et qui n'en revient pas" ("like a painter who watches the colors of day come alive under his fingers and can't get over it"). Note that lovely spoken idiom n'en revient pas, which translates to being utterly flabbergasted or amazed. By the end, Dassin reaches peak romantic obsession: he decides the entire secret of life, the grand "pourquoi" ("the reason why"), would simply be to create her out of thin air just so he could spend his days staring at her ("simplement pour te créer et pour te regarder"). It is pure, unabashed pop melodrama, delivered with total sincerity.