！！
Oh! There's a butterfly!
Literal
Ah! Butterfly [subject-が] exists!
あ is a short interjection of recognition or surprise — 'oh!,' 'ah!.' 蝶々 (chouchou) reduplicates the kanji 蝶 ('butterfly') for the colloquial form; the simpler 蝶 (chou) is more formal. Note いる (existence verb for animate beings) — butterflies, being living creatures, take いる, not ある.