Poner el cuerno literally translates to "to put the horn on". It's a colorful and common Spanish idiom used to say that someone is being unfaithful in a relationship.
In the song, the singer warns his ex, "Te pone el cuerno y no te enteras" (He cheats on you and you don't even find out). This dramatic accusation is his way of proving that she made a mistake by leaving him for her new partner.
This song is one big, brutally honest message to an ex. Carin León sees his former girlfriend with a new guy and he is not impressed. He's convinced she's just pretending to be happy, telling her, "Aparentando que estás bien / Deja de fingir que tú ya andas al cien" (Pretending you're fine / Stop faking that you're doing great). He thinks she left him for something better, but ended up with someone who doesn't even compare to him, saying the new guy "a mí no me llega ni a los zapatos" (doesn't even reach my shoes).
He doesn't hold back at all. The main point of the song is right in the title: "A ti ese vato no te queda" (That dude doesn't suit you). He claims this new boyfriend is cheating on her and she's just turning a blind eye: "Te pone el cuerno y no te enteras / O te haces la ciega" (He's cheating on you and you don't find out / Or you're playing blind). And why is she doing all this? In his mind, it's all just to get back at him: "Nomás pa' que me duela" (Just so it hurts me).