Arcoiris translates to "rainbow". It's a beautiful and evocative word that paints a vivid picture, and while not extremely rare, it's less common in everyday song lyrics than words like "amor" or "sol".
In the context of "Que Bonito Es Vivir", the word appears in the line "Si brilla el arcoiris o si nevando está" (If the rainbow shines or if it's snowing). It contributes to the song's theme of finding joy and beauty in all of life's moments, whether bright and colorful like a rainbow, or quiet and serene like snow.
The whole song circles one line: 'Qué bonito es vivir' [how nice it is to live]. Il Divo start with the sun itself waking up, 'Ha salido el sol' [the sun has risen], 'Y ha sonreído' [and has smiled], then keep stacking up reasons to feel that way, four voices layering into one big, unhurried harmony instead of building toward a chorus with a hook.
What's nice about it is how little the reasons matter. Rainbow or snow, it's the same: 'Si brilla el arcoiris' [if the rainbow shines], 'O si nevando está' [or if it's snowing]. The one thing that actually counts is 'Estando tú a mi lado' [with you by my side]; after that, 'A mí qué más me da' [what else does it matter to me]. By the last minute they've stopped adding new lines altogether and are just singing the title over and over, as if repeating it is the whole point.