Mariposas literally means "butterflies." While it's a common word, its use in this song is a beautiful example of a universal idiom.
In Spanish, "sentir mariposas en el estómago" (to feel butterflies in the stomach) is a common phrase to describe the nervous, fluttery feeling of excitement or love, just like in English. The song uses this imagery to convey the intense romantic feelings the singer has for the person they saw.
Camila built "Nada" around one plain confession, repeated until it stops sounding like an exaggeration: without you, I'm nothing. The verses set it up with the usual signs of falling hard, "Solamente pienso en ti" ("I only think of you") and "Siento mariposas por dentro" ("I feel butterflies inside"), then flip them. He calls it "Un beso que no existió" ("A kiss that didn't exist"), and she's still holding on: "Mientras tu me olvidas, yo intento prenderme a tu corazón" ("While you forget me, I try to cling to your heart").
The chorus, "Sin tu amor no soy nada, nada" ("Without your love I'm nothing, nothing"), turns the song from a breakup complaint into something closer to panic. By the bridge she's dropped the images altogether: "No puedo respirar, no se como seguir, porque este inmenso vació me hace así" ("I can't breathe, I don't know how to go on, because this immense emptiness makes me this way"). The plainer the words get, the worse it sounds for her.