Chassé-croisé literally describes a 'criss-cross' step in dancing where partners switch places. It's a fantastic and unique French term that you won't hear every day.
In the song, it's used metaphorically in the line "Amour chassé-croisé" (criss-cross love) to perfectly describe the show's tangled romance: Marinette loves Adrien, but as Chat Noir, he loves her superhero alter-ego, Ladybug. This one phrase captures the entire complicated, heart-wrenching dynamic.
This song is the theme for the animated series Miraculous: Tales of Ladybug & Cat Noir, and it perfectly lays out the show's entire plot! The singer, Lou, voices Marinette, who tells us she's just a normal girl: "Je m'appelle Marinette / Une fille comme les autres" [My name is Marinette / A girl like any other]. But when evil appears, destiny chooses her to become the superhero Ladybug. The song then introduces the show's central romantic problem, a complicated love square she calls an "Amour chassé-croisé" [criss-cross love]. Marinette's heart belongs to a boy named Adrien, not the masked hero who loves her, Cat Noir.
Then Lenni-Kim jumps in as the voice of Cat Noir, her crime-fighting partner. He describes his side of the story, explaining his fear of "aimer pour rien" [loving for nothing] because the girl he loves, Ladybug, can't love him back while she's busy being a hero. What makes this so fun is that they don't know each other's secret identities. Marinette is in love with Adrien, who is secretly Cat Noir. And Cat Noir is in love with Ladybug, who is secretly Marinette! It's a super fun, slightly frustrating story about two heroes saving Paris while being completely oblivious to who the other person really is.