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.
This song is a straight-up declaration of loyalty between two partners in crime. The main idea is right there in the title and the hook: "Yo por ti, tú por mí" (Me for you, you for me). ROSALÍA and Ozuna trade lines about being a powerful, successful duo who have each other's backs no matter what. They're living a life of luxury, "rodeá' de flores y billete'" (surrounded by flowers and bills), and they dare anyone to get in their way.
It's a classic "us against the world" story, but with a modern, glamorous twist. The commitment is intense. ROSALÍA says she'd let them put her in handcuffs for her partner: "que me pongan los grillete'" (let them put me in handcuffs). Ozuna is just as devoted, saying he'd spend all his money just to see her happy: "Lo que tengo lo gastaría porque tus ojos, ma'i, brillarían" (I would spend what I have so your eyes, girl, would shine). They see themselves as a legendary pair, "dos cantantes como los de antes" (two singers like the ones from before), whose mutual respect is measured in success and diamonds.