Firmamento is a poetic word for "the heavens" or "the sky". It's a much more literary and dramatic term than the common word cielo.
In this passionate song, Luis Fonsi makes a grand, romantic promise: "Te bajo el firmamento si es que así vas a creer" (I'll bring down the heavens for you if that's what it takes for you to believe). He uses this beautiful word to show the incredible lengths he would go to win back his love, making it a perfect example of romantic hyperbole.
This song is one big, desperate message to an ex, delivered through a mutual friend. Luis Fonsi kicks things off by asking, "Amigo, si la ves / Dile que tú me viste diferente" (Friend, if you see her / Tell her you saw me looking different). He's putting on a brave face, "jugando a ser feliz" (playing at being happy), but he wants her to know he's miserable inside. He admits he messed up completely and is begging her to just pick up the phone so he can explain.
His main plea is a huge promise: "Dile que cambiaré, yo cambiaré" (Tell her I will change, I will change). He swears he'll fix whatever it is he does wrong and makes some pretty dramatic declarations, like "Te juro que vivo por ella / Y por ella me moriré" (I swear I live for her / And for her I will die). The second half of the song turns into a direct apology, repeating "Por favor, perdóname" (Please, forgive me) over and over. It's a classic pop song about regret and the hope for a second chance.