Maestro means "teacher" or "master." While it's a common word, its use here is particularly striking and metaphorical.
In the song, David Bisbal sings, "Fue nuestro amor, mi máximo maestro de dolor" (Our love was my greatest teacher of pain). He personifies love as a profound instructor, highlighting how the experience of a past relationship, despite its pain, taught him valuable lessons. This poetic usage elevates the word beyond its literal meaning, making it memorable and emotionally resonant.
David Bisbal turns a breakup into a physics lesson on "Fue Nuestro Amor." He tries to calculate the damage and pin down exactly what broke — "Cómo calcular los daños en su dimensión" ("How to calculate the damage, its whole dimension") — before landing on the hook that blames the relationship itself: "Fue nuestro amor, mi máximo maestro de dolor" ("It was our love, my ultimate teacher of pain"). He stacks a run of opposites to sell the intensity of it — "El cielo y el infierno en oposición" ("Heaven and hell in opposition"), "El fuego y el papel en colisión" ("Fire and paper in collision"), "Salto mortal del corazón" ("A death-defying leap of the heart") — before the final verse closes the door: "Te dejo atrás, ya te solté" ("I'm leaving you behind, I already let you go").