Grillete means "shackles" or "handcuffs". It's a powerful, dramatic word that you don't often hear in a love song, making it instantly intriguing.
In the lyric "Yo por ti, tú por mí, que me pongan los grillete'" (Me for you, you for me, let them put the handcuffs on me), it's used as a bold metaphor for a 'ride-or-die' loyalty. It expresses a love so strong that the singers are willing to face any consequences, even being arrested, as long as they are together.
"Yo X Ti, Tu X Mi" is all about total, ride-or-die loyalty. The title repeats like a mantra: "Yo por ti, tú por mí" [I'm there for you, you're there for me]. ROSALÍA, the superstar from Spain, teams up with Puerto Rican singer Ozuna to play two big-time artists in an unstoppable pact. Right away, she flexes with jewelry and cash: "Colgando del cuello los juguete' / Rodeá' de flores y billete'" [Toys hanging from my neck, surrounded by flowers and bills]. Here, "juguete'" [toys] is street slang for flashy luxury chains and watches. She warns anyone who might step in their way with a blunt "'bang-bang' si con nosotros te entrometes" ['bang-bang' if you mess with us], betting everything on their bond and even daring them to lock her up: "que me pongan los grillete'" [let them put the shackles on me] if she falls.
What makes the track stick is how they compare themselves to old-school greats: "Somos dos cantantes como los de antes" [We are two singers like the ones from back in the day]. Instead of empty sweet-talk, they measure respect in sold-out stadium tickets and diamonds ("El respeto en boletos y diamantes"). Ozuna jumps in, admitting that his heart literally skips a beat ("se me para el cora'") just looking at her, ready to spend every dime he owns simply to watch her eyes light up. They sound thrilled that this exact groove can blow up from global stages all the way to a random street corner ("Que hasta en la esquina esta canção sonaría"). It is loud, wealthy, and completely dedicated to keeping each other safe.