Greñas is a colloquial Spanish word for 'shaggy' or 'messy hair'. It's a more informal and expressive alternative to the standard words pelo or cabello.
In the song, Ivan Cornejo sings the powerful line, "Arrancas mis greñas cuando pienso en recuerdos" (You make me want to rip my hair out when I think of memories). This is a visceral metaphor for extreme frustration and mental anguish, vividly capturing the feeling of being so overwhelmed by painful memories that you want to physically pull your hair out.
This song is pure, unfiltered heartbreak. Ivan Cornejo sings about the moment you realize it's truly over and you have to accept it, even if you don't want to. He's looking back after a breakup, feeling like his whole world has changed. His weekends don't even sound the same anymore. He admits his ex's love was fragile, like it was "hecho de papel" [made of paper], and the worst part is that she's already in love with someone else.
The real gut-punch of the song is how mismatched their plans for the future were. He sings, "Lo que más duele / Es que yo te vi / En mi futuro / Pero yo no en el tuyo" [What hurts the most / Is that I saw you / In my future / But I wasn't in yours]. The pain he feels is so intense it's physical. He describes her memory as something that tortures his body and makes him want to pull his hair out. He feels like she stole a part of him, and all he can do now is accept the painful truth: "Ay, baby, ya te perdí" [Oh, baby, I've already lost you].