Rebuznar means "to bray," which is the loud, harsh cry of a donkey. It's a very specific and somewhat humorous verb that you don't often find in song lyrics.
In this song, Joan Sebastian uses it to express his self-deprecating despair, questioning whether his singing is even worthy of being called singing, or if it's just the pathetic sound of a donkey. This vivid imagery makes the word memorable and highlights the raw emotion in the lyrics.
In "Que Dios Bendiga," Joan Sebastian is saying goodbye to a love that's leaving him for good. He's pretty blunt about it, even calling himself "menos inteligente que un cuadrúpedo" [less intelligent than a quadruped] because he's still singing about it. It’s a mix of sadness and a bit of self-deprecating humor as he watches her go, knowing she won't come back, even though he still loves her.
He wishes her well, saying "Que Dios bendiga tu camino" [May God bless your path] and "Que Dios bendiga tu sendero" [May God bless your trail]. But there's a sting in his blessing, too, as he adds, "me olvidarás, ya no no te mueres sin mí" [you will forget me, you won't die without me]. It's a powerful and honest look at heartbreak, where he's both accepting the end and a little bit bitter about it.