Resuelve is a clever bit of slang you won't find in your textbook. It comes from the verb resolver (to solve), but here it's a noun describing a person who is just a "solution" for a temporary, casual need—not a serious partner.
In the song, Becky G asserts her value by singing, "En tu mente no soy un resuelve" (In your mind, I'm not just a fling). It's a powerful and modern way of saying she's not just a temporary fix and demands to be taken seriously.
Becky G is calling someone out in this song, and she is not holding back! She's talking to an ex who claims he's over her, but she knows he's lying. She's so sure of herself, she just dismisses his words: "Para qué decir mucho / Ya ni escucho cuando dices que tú me olvidaste" (Why say so much / I don't even listen when you say you've forgotten me). The whole song is built around this idea, summed up in the title line: "No digas que no si sí" (Don't say no if it's yes).
She lays out all the proof that he's still into her. She knows that "Si te llamo, tú vienes donde mí" (If I call you, you come to me) and that even if he doesn't miss her right now, he will soon. She's confident and in control, reminding him that she's not just some casual hookup, or a "resuelve" (a quick fix). But even with all this confidence, she leaves the door open for him, telling him not to pretend and that "La puerta está abierta por si quieres entrar" (The door is open if you want to come in). It's a power move in a complicated relationship.