It’s a strip of fabric gathered and stitched to a garment’s hem, usually to a skirt.
The ruffle, or frill, can be made of the same fabric or of a different material.
During the 19th and 20th century ruffles were used to adorn dresses and skirts, specially of evening garments.
Check out:
The Post’s Pinterest Board: https://br.pinterest.com/fashionsummedup/r/ruffle/
The Post’s Brazilian Portuguese version: https://amodaresumida.com/babado/
Bibliography: Callan, Georgina O’Hara; Enciclopédia da moda de 1840 à década de 90 / Georgina O’Hara Callan ; verbetes brasileiros Cynthia Garcia : tradução Glória Maria de Mello Carvalho, Maria Ignez França – São Paulo : Companhia das Letras, 2007.