This is a skirt that is drawn together to form a puffy effect on the clothing.
Gathered skirt can be done on knitwear or other textiles.
In machine knitting, a gathered design is made by transferring two or more stitches to each needle and knitting them, this creates a ruffling or puffing effect.
In hand-knitting, a gathered design is made by knitting into a stitch twice.
In crocheting, a gathered design is made by knitting two or more stitches into a stitch.
See the video below.