Caiga means "falls", but it's a special form of the verb 'caer' (to fall) called the subjunctive mood. It's used here to talk about a hypothetical, but possible, event: "aunque... el cielo caiga" (even if... the sky falls).
This dramatic and poetic line shows the singer's unwavering devotion, promising to stand by his love no matter what happens. Learning the subjunctive is a major step in mastering Spanish, and this song uses it to create a powerful, unforgettable image.
Prince Royce takes a familiar classic and brings it straight into the world of bachata, mixing English and Spanish throughout the whole track. The message is simple and direct: as long as you stay close, nothing else is scary. He sings about dark nights where the moon is the only light in front of him ("Y la luna es la luz que brilla ante mí" [And the moon is the light that shines before me]), promising that he will not cry ("No lloraré" [I won't cry]) even if mountains collapse or the whole sky falls down ("Y aunque las montañas o el cielo caiga" [And even if the mountains or the sky falls]).
What makes this version fun is how naturally he switches languages mid-thought. He flips between "Miedo no, no tendré" [Fear, no, I won't have it] and "I won't, me asustaré" [I won't be scared], pairing the English phrase "stand by me" directly with its Spanish counterpart, "junto a mí" [next to me / by my side]. Royce even interrupts his own romantic mood to hype himself up, dropping lines like "That's class right there" and shouting out his own name: "What's my name? Royce." It is warm, sweet, and packed with playful confidence.