Puñeta is a strong exclamation in Spanish, similar to "damn it" or "heck" in English, but it can carry more intensity depending on the context and region. It's often used to express frustration, anger, or surprise.
In this song, Arcángel uses it in the line "Sin entender por qué puñeta' no te suelto" (Without understanding why the heck I don't let you go). It highlights his deep confusion and exasperation over his inability to move on from a past relationship, making it a very emotionally charged and memorable word in the lyrics.
Arcángel spends "En Tus Sueños" pretending he's fine and failing at it in real time. He opens like a man who's moved on: "Te buscaste a otro, esa mierda ya ni me asombra" [You found someone else, that doesn't even surprise me anymore]. Cool enough, until the next lines undo it.
By the second verse he's asking the question that actually matters: "¿Y así de fácil ya pasaste al siguiente?" [And that easily you already moved on to the next one?]. That's not indifference, that's disbelief. He admits it outright two lines later: "Sin entender por qué puñeta' no te suelto" [Without understanding why the hell I can't let her go]. The title does the rest of the work; if he can't have her awake, she gets to him asleep. "Tú eres ese problema que nunca he resuelto" [You're that problem I've never solved] is as honest as breakup rap gets, and it's over before the beat even builds.