Sirena literally translates to "mermaid". It's a beautiful and somewhat magical word that you won't find in every Spanish pop song.
In "Otra Vez", the singer uses sirena as a poetic compliment for a woman who reappears in his life. The line "Hoy vuelve a aparecer bajo la luna llena, sirena" (Today you appear again under the full moon, mermaid) paints a picture of an enchanting, almost mythical woman, making it a much more captivating compliment than simply calling her beautiful.
Ever run into an ex and suddenly all the old feelings come rushing back? That's exactly what's happening in "Otra Vez". Zion & Lennox and J Balvin find themselves face-to-face with a past love, and the connection is immediate. The singer is a little suspicious, asking, "Después de tanto, me buscas / ¿Por qué será?" (After so long, you look for me / Why could that be?). He knows this person has the power to completely disrupt his life, to "volverme a poner el mundo al revés" (turn my world upside down again).
Instead of running away, he leans right into the chaos. He remembers their wild past of endless party nights that felt like a fantasy but were very real. He sees this reunion not as a random event, but as destiny, saying, "No es casualidad que nos viéramos en la misma disco" (It's not a coincidence that we saw each other in the same club). The plan? To dive back in and relive that intense connection, starting right now.