A variation on ruffly / frilly yarn - it’s a strip of fabric with a serged-on border, which is the edge for knitting.
It is available in a variety of prints.

54% Polyester (fabric)
47% Acrylic (yarn)

6 sts / 12 rows = 10 cm on 8mm needles

Hand wash, warm. Do not bleach. Dry flat. Do not iron. May be dry cleaned.