An onomatopoeic word describing something thin fluttering or flapping (like cloth or paper); can be used adverbially or as a noun/suru-verb.
風でスカートがピラピラして落ち着かなかった。
The skirt kept fluttering in the wind and I couldn't relax.
机の上のチラシがピラピラと床に落ちた。
The flyer on the desk fluttered down to the floor.
彼女は舞台衣装の端をピラピラさせながら踊った。
She danced while letting the edge of her costume flutter.