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 out an ex who acts completely unfazed in public but runs back the second she hits him up. The whole track revolves around the cheeky hook "No digas que no si sí" ["Don't say no when it's really yes"]. She knows his tough act is pure fiction. When she sings "En tu mente no soy un resuelve" ["In your mind I'm not just a rebound/quick fix"], she uses the Caribbean slang term resuelve—someone you call only for convenience—to point out that she means way more to him than an easy distraction. Even if he denies missing her today, she laughs it off with "ahorita sí" ["you will very soon"], because one text from her is all it takes to break his resolve.
She is not sitting by the phone waiting, though. Becky reminds him that her bag is fully secured without his help: "Mi cartera sigue llena igual sin ti" ["My wallet stays just as full without you"]. While her door is unlocked if he wants to visit, she lays down the ground rules. He lost the right to act jealous if other people flirt with her, and if he gets too bold with his lies, she tells him flat out: "Bájale, bebé" ["Tone it down, baby" / "Take it down a notch"]. It is a fun, cocky track about having total upper-hand power over an ex who simply cannot stay away.