Ave de paso literally translates to "bird of passage". It's a poetic expression used to describe a transient person, someone who comes and goes without settling down, much like a migratory bird.
In this classic ballad, the singer describes his lover this way, setting a bittersweet tone for the song. He fell in love with someone he knew would eventually fly away, beautifully capturing the feeling of a fleeting yet impactful romance.
This is a massive power ballad about wanting someone back with an almost overwhelming intensity. The story starts with a relationship that was maybe supposed to be casual. The singer says, "Fuiste ave de paso" (You were a passing bird), but then he got used to her, and they "inventamos la aventura del amor" (invented the adventure of love). But just as his life was full, she left without even saying goodbye.
Now, he's left with an intense longing. The chorus hits with the very direct line, "Hoy tengo ganas de ti" (Today I want you). He doesn't just miss her, he wants to "apagar en tus labios la sed de mi alma" (quench the thirst of my soul on your lips). It's a hugely dramatic and passionate way to express how much he needs her back. The song is pure heartache and desire, describing the silence and pain of knowing he's lost her.