Mariposas literally means "butterflies." While it's a common word, its use in this song is particularly evocative and universally understood as a metaphor for the fluttery, nervous, yet exciting feeling of new love or intense attraction.
In the lyrics, "Mariposas que se revuelven cuando me rozas" (Butterflies that stir when you touch me), it perfectly captures that classic sensation of having "butterflies in your stomach." This makes it an engaging and relatable word for anyone experiencing the thrill of romance, making it a great hook to explore the song's theme.
Bely Basarte opens by throwing out the whole script of a normal relationship: "Contigo yo no quiero ni flores ni canciones" (With you I don't want flowers or songs). She'd rather skip the plans and just go, "Quiero viajar sin frenos y escapar a media noche" (I want to travel without brakes and escape at midnight). The title shows up a few lines later, right in the moment a touch does something to her: "Mariposas que se revuelven cuando me rozas" (Butterflies that stir when you touch me), and for a second the whole room seems to hold still and watch.
By the chorus the song has stopped being coy about what it wants. The jeans are the first thing to go, "Que nos sobren los vaqueros" (Let our jeans be unnecessary), and the two of them end up on the floor instead of the bed. What she's actually asking for is total coverage: "Que no quede ni un solo palmo en nuestra piel" (That there isn't a single inch on our skin), "Sin dosis de amor" (Without a dose of love). It's a song built entirely around that one physical spark, not around anything either of them plans to say about it later.