Desierto means "desert." While it's a common word, its use in this song is particularly striking and metaphorical.
Diego Torres sings, "Como agua en el desierto" (Like water in the desert), comparing the sudden appearance of a loved one to a life-saving oasis in a barren landscape. This vivid imagery makes the word memorable and highlights its profound meaning in the context of finding hope and solace.
Diego Torres opens this one lost, walking with no direction: 'Caminaba solo y confundido' [I was walking alone and confused]. Then someone shows up out of nowhere, and the whole song turns on one image: 'Como agua en el desierto' [Like water in the desert]. It's not a grand metaphor about love conquering everything. It's relief, the specific feeling of finally getting what your body needed.
That's the trick of the song. It never claims the relationship worked out. She barely told him anything about her life, he still doesn't know where she is, and by the bridge he's stuck in the same place: 'Aun te espero y aun no puedo dejarte ir' [I still wait for you and I still can't let you go]. He calls her 'el mejor remedio a mi soledad' [the best remedy for my loneliness], a strange thing to say about someone who left. This isn't a song about winning her back. It's about one person passing through fixing something in you, then leaving before it heals.