Suplante comes from the verb suplantar, meaning 'to supplant' or 'to replace'. It's a more powerful and less common word than reemplazar, often implying an illegitimate taking of someone's rightful place.
In the song, Ozuna sings, "No sé si llegará alguien que te suplante" (I don't know if someone will ever come who can replace you). He uses this word to express the profound fear that no one could ever fill the space his lost love occupied, highlighting how unique and irreplaceable she was.
Puerto Rican reggaeton star Ozuna kicks off "Una Flor" by telling us it is based on real life, and you immediately feel the sting of an ended relationship. He compares the painful death of his romance to watching a rare bloom wither away, singing that he has seen "morir una flor / del único color / que no creo ver jamás" [a flower die of the only color I don't think I'll ever see again]. Instead of pretending he did nothing wrong, he admits his own flaws right alongside his ex's mistakes. He confesses, "Te he mentido, he pecado igual que tú" [I have lied to you, I have sinned just like you] and uses the common idiom "cargué mi cruz" [I carried my cross] to show he took on his share of suffering and burden from their drama.
The imagery gets intensely personal as Ozuna holds onto the physical traces of his past love. He says that if the sky turns cloudy, "el perfume de tu ropa interior queda conmigo" [the scent of your underwear stays with me], and promises to keep her photo tucked under his pillow even as he grows old. It almost feels like a ghost story in the opening lines where someone knocks at the door or speaks while you sleep, but cannot be seen. In the end, his attachment goes beyond this life: he asks God that even if his soul flies far away after death, he can still watch over her.