Vaivén literally translates to swaying or to and fro, describing a repeated back-and-forth motion. It is an evocative word that captures a sense of constant movement and emotional uncertainty.
Alfredo Olivas uses it to describe the push and pull of a secret relationship, singing "Y en el vaivén de lo que está y no permitido" (And in the swing of what's allowed and what's not), showing the constant tension of forbidden love.
"Vaivén" is about a love that only gets to exist in secret, in the back-and-forth ("vaivén") between what's allowed and what isn't. The singer only reaches out when the timing feels safe, when "the coast is clear" and the waters have calmed, and he admits he lowers his voice even just saying how much she means to him.
Even so, he treats her as the best part of his day: the last thing on his mind before sleep, the first thing when he wakes up, the song that comes on over drinks and pulls him straight back to the day they met. He calls her his favorite sin, the kiss he can't do without.
By the end the song turns into a toast, celebrating music, tequila, and the two of them together, daring nothing to go wrong.