En el acto is a powerful Spanish idiom that literally translates to "in the act", but it's used to mean "instantly" or "on the spot". It's a more dramatic and emphatic way of saying inmediatamente.
In the song, the line "Me enamoré de inmediato, eso fue en el acto" (I fell in love immediately, it happened on the spot) uses this phrase to perfectly capture the sudden, electrifying feeling of love at first sight. It's a fantastic expression to learn to make your Spanish sound more natural and dynamic.
Have you ever looked at someone across a room and lost your cool completely? That is the exact spark behind "Te Vi" by Colombian group Piso 21 and Micro TDH. The singers admit that the instant they caught sight of their crush—"Y cuando yo te vi / Me enamoré de inmediato" (And when I saw you / I fell in love immediately)—they fell head over heels on the spot. From sharing a casual kiss on the dance floor ("Nos dimos par de picos" [We gave each other a couple of pecks]) to losing all track of time when holding them close, the whole track is an upbeat confession about being totally hooked.
What makes the lyrics fun is how honest they are about being nervous. One minute the singer asks for a little spin on the dance floor ("Dame una vueltica y empieza a bailar" [Give me a little spin and start dancing]), and the next, he admits that just the thought of her leaving makes him physically shake: "Tú me gustas tanto, que si tú te vas / Me pongo nervioso y empiezo a temblar" (I like you so much that if you leave / I get nervous and start trembling). All other distractions vanish, and the only goal left is simple: "Quiero hacerte pasar un buen rato" (I want to give you a good time).