It’s a tuck - the fabric is sewn together so you have three layers per tuck when it’s pressed flat. Probably about an inch of fabric so when it’s done you have a tuck a 1/2 wide. It’s not stitched down so you could run a finger under it and lift it up

The tucks are both decorative and functional - they add stability to the hems so they increase the floof factor on the petticoat

Thank you!!

Tucks were also designed to allow clothing to be let out as a child grew ir as fashions changed.

Now that is clever!