Fruto prohibido literally translates to "forbidden fruit". This phrase is a powerful biblical allusion, immediately evoking a sense of temptation, danger, and something highly desired but unattainable.
In the context of the song, the narrator discovers that their long-lost love is now "fruto prohibido" because she is with another person. This makes their love illicit and impossible, adding a dramatic and poignant layer to the story of their reunion.
The whole song is one man doing arithmetic on a disappearance. They were engaged, "Nos íbamos a casar, de repente" (we were going to get married, and suddenly), and then she was gone and nobody would let him near her. He had no address for her, so he did the only thing left to do: "Siempre te mandé cartas y cartas" (I kept sending you letters and letters), fired off at every place he could think she might be, and every one of them came back to nothing.
The letters did work, just far too late. "Un día una de esas cartas cayó en tus manos" (one day one of those letters fell into your hands), and the answer he finally got was the one he had never considered: "Que eras fruto prohibido" (that you were forbidden fruit), living with somebody else. Los Ángeles Azules keep the accordion bright and bouncing underneath all of it, which is the trick this band has always pulled. You can dance the entire way through a story about a man who spent years writing to an address that was never going to be his.