Vaivén literally translates to "swing" or "back and forth movement." It's a less common word in everyday Spanish, making it interesting and memorable.
In the context of the song, Wisin uses vaivén to describe the emotional push and pull he feels with someone who comes and goes, creating a sense of uncertainty and playful frustration. He sings, "Me tienes en un vaivén, vaivén," meaning "You have me in a back and forth," perfectly capturing the dynamic of their on-again, off-again relationship.
Wisin builds the whole song out of one two-line hook, and it never resolves: "Se viene de noche" / "Se va de día" (it comes at night, it leaves by day) lands sixteen times. Early on he stops describing her and starts addressing her — "Te vienes de noche" / "Y te vas de día" (you come at night, and you leave by day) — and admits that is all he has: "Si eres mía, no sé" / "Si me quieres, tal vez" (whether you're mine, I don't know; whether you love me, maybe).
The verses say what waiting on that schedule costs him. "Llevo detrás de ti par de meses" (I've been after you for a couple of months) opens the first one, and by the second he has handed over the steering entirely: "Llega sin avisar y me empieza a mirar" (she turns up without warning and starts looking at me), "Me tienes en un vaivén, vaivén" (you've got me swinging back and forth). Los Legendarios and Gangsta hold the beat on an even pulse underneath, so the only thing moving is him.