costilla literally translates to "rib". It's a common anatomical term, but Bad Bunny uses it here in a very clever and thought-provoking way.
He references the biblical story where Eve is created from Adam's rib, singing "La mujer no salió de la costilla del hombre / Fuimos nosotros que salimos de su pelvis" (Woman didn't come from man's rib / We came from her pelvis). By using "costilla", he immediately grabs the listener's attention to challenge an ancient narrative and make a powerful modern statement about the origin and strength of women.
Bad Bunny starts this song by talking about himself, but it quickly becomes about so much more. He traces his own path from being an altar boy to a huge trap star, admitting he's done things for money and has never even been loyal to his barber. He says he's a "super estrella, pero humano primero" (superstar, but human first) and that his Puerto Rican roots are what make him a winner. He's proud of his success and how it forces his competition to be better.
But then, the song's focus shifts completely. Bad Bunny gets angry about the state of the world, specifically racism and injustice. He says his good day was ruined by news of a shooting and a president who does nothing. He directly calls out the double standard where "Un negro con pistola, ya eso es un criminal / Pero un blanquito se la engancha y dicen que es un hobby" (A Black man with a gun, that's a criminal / But a little white guy straps one on and they say it's a hobby). He argues that people get upset about him winning a 'Composer of the Year' award but ignore the real problems, like police brutality and violence against women. He ends by saying if he could change the world, he would, but for now, the best prize is to "Poder ser yo, dejarte ser tú" (To be able to be me, and let you be you).