The word "mariposas" literally means "butterflies." While it's a common word, its use in this song is particularly evocative and culturally significant.
In Spanish, the phrase "tener mariposas en el estómago" (to have butterflies in the stomach) is a common idiom for feeling nervous or excited, especially in love. Gloria Estefan uses "mariposas escondidas" (hidden butterflies) to beautifully convey the subtle, fluttering feeling of new love and excitement that comes "cuando hay amor" (when there is love).
Gloria Estefan builds this song out of small, checkable evidence that someone is in love. "Hay mariposas escondidas" (there are hidden butterflies), the colors start shining, and then the line that does the real work: "Ya no existen las mentiras" (lies no longer exist). The chorus keeps returning to a flat, almost stubborn claim, "No hace falta nada más, hay amor" (nothing else is needed, there is love), and she never softens it.
The best verse is the one about paperwork. "Para que un papel firmado" (what is a signed paper for), "Testamentos arreglados" (wills all settled) asks why anyone bothers with contracts, since "La bondad es natural cuando hay amor" (kindness comes naturally when there is love). Then she drops the argument altogether and sings "Lalala-la-lalala" for most of the last minute, which is the honest ending for a song that already told us "sobran las palabras cuando canta el corazón" (words are unnecessary when the heart sings).