Espalda literally means "back" (as in, a body part). However, in this song, it's used in the common Spanish idiom "dar la espalda," which means "to turn one's back on someone."
Dalvin La Melodía uses this phrase repeatedly to express betrayal and abandonment, singing, "Porque me diste la espalda cuando te busqué" (Because you turned your back on me when I looked for you). It's a powerful way to convey heartbreak and disappointment, making it a memorable and emotionally charged word in the context of the lyrics.
Dalvin La Melodía spends this whole song asking one question that never gets an answer: "¿Por qué me diste la espalda?" [Why did you turn your back on me?]. What makes it sting is the line he puts right before it every time, "Si tú decías que me amabas" [If you said that you loved me], so the promise and the betrayal keep arriving in the same breath.
The heartbreak is physical here rather than poetic. It opens with "La pena enreda y atraviesa mis extremidades" [Sorrow tangles and pierces my limbs], and after that he stops sleeping, "Las noches se hacen largas, ya no tienen fin" [The nights become long, they no longer have an end], and stops eating, "No tengo ganas de nada, ni de comer" [I don't feel like doing anything, not even eating]. His wish is oddly practical for a breakup song: "Regresar al pasado y borrar la fecha" [Go back to the past and erase the date] they met. Then he turns away from all of it to call on his band, "Suénala, Mikey, suénala" [Play it, Mikey, play it], and the bachata guitar takes over. It is the one bit of the track that sounds like anyone is having a good time.