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 a romantic context. The artist sings, "Pero ellas me matan mis mariposas" (But they kill my butterflies), implying that other relationships extinguish the special, nervous excitement he feels with the person he truly loves. This makes the word a powerful symbol of genuine affection and unique connection.
Nanpa Básico's "Cuando Vienes" is a really honest look at a complicated love. He's clearly been through a lot, saying he's "roto de costumbre, consumiendo cosas" [broken from habit, consuming things] and has tried being with other people, but they just "matan mis mariposas" [kill my butterflies].
But then she comes along, and it's like everything changes. He loves it "cuando vienes limpia" [when you come clean] and how she "salvas la vida con tu luz de diosa" [save my life with your goddess light]. He appreciates that she just shows up, kisses him, and takes away his sadness without needing explanations. Even though he admits his life is difficult and he gives "más problemas que regalos, vivo en crisis" [more problems than gifts, I live in crisis], he still misses her every second, even if he doesn't say it. He even says he hates himself "por maldito y por lo que hice contigo" [for being cursed and for what I did to you]. It's a raw confession of regret and longing, accepting her just as she is, "incompleta o como vengas" [incomplete or however you come].