Inmensidad translates to "immensity" or "vastness." It's a beautiful and evocative word that paints a picture of something boundless and grand.
In "Piel Canela," the singer uses it to describe the vastness of the sea, contrasting it with the even greater immensity of his sadness if he were to lose his love. This poetic comparison elevates the emotion of the song, making it a memorable and impactful word choice.
Cepeda spends the first half of this bolero raising a bet nobody asked him to make. The sky can lose its stars, the sea its size, the rainbow its colour, the flowers their scent — Si perdiera el arco iris su belleza / Y las flores su perfume y su color (If the rainbow lost its beauty / And the flowers their perfume and their colour) — and none of it would cost him what aquella de quedarme sin tu amor (that of being left without your love) would. Two things are exempt from the trade, and both are small and physical rather than cosmic: Pero el negro de tus ojos que no muera / Y el canela de tu piel se quede igual (But let the black of your eyes not die / And the cinnamon of your skin stay the same). That second line is the title — piel canela, cinnamon skin.
Then the argument stops and the song just repeats a pronoun. Me importas tú, y tú, y tú / Y nadie más que tú (You matter to me, and you, and you / And nobody but you) — tú lands twelve times across those four lines, which is the whole point: after all the accounting with stars and oceans, the only word left is the one that means you. Bobby Capó wrote it, and it has been sung across Latin America ever since; Cepeda takes it unhurried and lets the guitar carry what the lyric has stopped arguing.