Problemón is the augmentative form of problema (problem), making it a 'big problem' or 'huge mess'. It's a very colloquial and expressive word that instantly sounds dramatic and engaging.
In the song, Álvaro Díaz and Rauw Alejandro use it to describe a secret, forbidden relationship. The whole affair is a problemón because it has to be kept hidden from everyone, especially the girl's boyfriend, which creates a tense and exciting vibe throughout the lyrics.
This song is all about a secret, forbidden relationship that's so intense it's a problemón [huge problem]. Álvaro Díaz and Rauw Alejandro lay out the rules for this affair: "No le puedes decir a nadie" [You can't tell anyone] and "Solo textea si es tarde" [Only text if it's late]. It's clear this isn't supposed to be happening, but they just can't stop themselves, asking, "Pero dime cómo paro lo de tú y yo" [But tell me how I stop this thing between you and me].
The whole vibe is risky and exciting. They know if anyone finds out, it will be a papelón [a big embarrassing scene]. Rauw Alejandro's verse adds a funny, modern twist to the story. He describes being in her room when Netflix asks if they're still watching, and then the real drama hits: "Tocando la ventana, su novio estaba afuera" [Knocking on the window, her boyfriend was outside]. It's a close call that shows just how dangerous their secret is.