Esclavo literally means "slave". It's a powerful and intense word you don't often encounter in songs, used here to express complete emotional surrender.
In this ballad, the singer says, "Soy esclavo a tu mirar" (I'm a slave to your gaze). He uses this strong metaphor to describe being so captivated by someone's look that he feels powerless, perfectly capturing a feeling of hopeless devotion.
This song is a straight-up heartbreak anthem for when you feel like you did everything right, but it all went wrong anyway. The singer is completely lost after a breakup, asking his ex, "Dime en qué te fallé" (Tell me where I failed you). He feels like he's getting nothing but hate from her, and he has no idea why. He even says, "No sé qué hice pa' merecer / El odio que por mí tienes" (I don't know what I did to deserve / The hate you have for me). It's a really direct and honest look at the confusion that comes when a relationship ends without clear answers.
He's not handling it well at all. His only solution has been drinking, and he admits he can't stop loving her even though she's already with someone else. He begs her to tell her new partner that he's still in her heart, swearing, "Yo tengo a Dios como testigo / Nunca te traicioné" (I have God as my witness / I never betrayed you). The song follows his desperate attempt to understand what happened and his inability to let go, making it a perfect soundtrack for a lonely night.