Veneno literally means "poison". It's a powerful and evocative word used to describe something toxic or harmful.
In this song, the artist uses this word metaphorically to express the conflicting nature of a painful love affair. He sings about a "veneno dulce" (sweet poison), perfectly capturing the feeling of being addicted to a relationship that is both intoxicating and destructive.
This song lays out one of the most intense metaphors for a breakup you'll ever hear. The singer isn't just heartbroken; he's going through withdrawal. He compares his ex to a drug he can't stop using, singing, "Eres la droga que ya no puedo dejar" [You are the drug I can't quit]. He's trying to numb the pain of missing her with other things, but it's not working. He admits, "Ya traigo los ojos rojos de tanto que fumé / Y de tanto llorar" [My eyes are already red from smoking so much / And from crying so much].
The whole song explores this idea of a destructive, all-consuming love. He knows it's bad for him, saying she's like "el veneno dulce que se quedó en mi piel" [the sweet poison that stayed on my skin]. He even recognizes the self-destructive pattern with the line, "Uno siempre es adicto a lo que lo destruye" [One is always addicted to what destroys them]. It's a powerful and honest look at how a person can feel completely dependent on someone, even when the relationship is over and causing nothing but pain.