Amargura literally translates to "bitterness". It's a powerful emotional word that perfectly captures a feeling of sadness mixed with resentment or disappointment.
In the song, which is also titled "AMARGURA", KAROL G sings about the pain of seeing her ex happy with someone new. The key line, "qué amargura me da" (what bitterness it gives me), is the heart of the track, expressing the deep sorrow and heartbreak she feels despite trying to act like she's okay.
Have you ever seen your ex looking happy with someone new? That's the feeling KAROL G sings about in "AMARGURA." She sees her old flame kissing someone else and pretends she's fine with it. She even says, "Por fuera me río, pero por adentro estoy rota" (On the outside I laugh, but on the inside I'm broken). It's that classic feeling of putting on a brave face when you're actually falling apart.
She gets into all the painful little details of a breakup. She misses him, feels the emptiness of her room, and hates being left on read. She has this one amazing line: "Estaría tomándomelos contigo y no por ti" (I'd be drinking with you and not because of you). The song ends with a very blunt warning: "Esta canción / Que no me la pongan borracha, hijuepta" (Don't play this song for me when I'm drunk, motherfcker). It's the kind of song you scream along to when you're sad, but also kind of laughing.