Amarrada literally means "tied down" or "moored" (like a boat). However, in the context of relationships, it's a colloquial term meaning "committed" or "taken."
In this song, the singer uses it to highlight the complicated situation: "Yo estoy soltero, tú amarrada" (I'm single, you're taken). It's an interesting and common way to describe someone in a committed relationship, especially when there's a secret affair involved, making it a key word for understanding the song's central conflict.
Hozwal and Jay Wheeler team up for "Cual de los Dos" [Which of the Two], a reggaeton track that dives into the messy world of a secret affair. Our singer is totally fed up with being the other guy, the one she prefers in her mind but not in her bed. He's tired of the games and the sneaking around, asking her directly, "Dime, con cuál de los dos / Porque ya no estoy pa' tu juego" [Tell me, with which of the two / Because I'm not here for your game anymore].
He's clearly smitten, describing how he fell for her lips and the way she looks and walks. But he knows it's a dead end: "Yo estoy soltero, tú amarrada" [I'm single, you're tied down]. The escapes and secret rendezvous are fun, but the pain of it ending and her going back to her partner is too much. He calls her out on her double life, asking if she really enjoys hiding their love, even for something as simple as going to the movies. It's a blunt question: "¿Está en tus planes seguir este juego?, ¿piensa' que termine?" [Is it in your plans to continue this game?, do you think it will end?]. He's ready for her to choose, because he's tired of losing himself in a love that has to stay hidden.