The life cycle of the butterfly is an important symbol of the novel to which, and of course, the title alludes.
Literally, butterflies (in Spanish, "Las Mariposas") are beautiful but have very brief lives. The same is true of the Mirabel sisters, Minerva, Maria Teresa, and Patria, beautiful women whose young lives are cut short.
But strength and beauty are not the only traits butterflies share with the sisters. Despite their seeming physical weakness, butterflies are able to endure hardships that seem incredible (like flying hundreds of miles.) And, despite their seeming lack of power in the patriarchal regime of Trujillo, the Mirabels find astonishing strength despite their outward delicacy.
There are other important symbols, like Mate's hair ribbon and her braid, which, once her source of feminine pride, later is used to conceal contraband, and the "crown of thorns" that Dede tends in her garden, symbolizing her own lifelong torment.