I'd guess it's a box-pleated trim attached around the edges of a skirt.
But what really matters is what you want to do. Are you trying to replicate this exact costume ? Is there another reference image of something else if not ? If it's an original design you can consider any option that pleases you and is what you'd like to do.
Disclaimer : This post written by a Grumpy Old Man
Actually in the pipes:
- Remaking Ser Loras doublet.
- Luke Skywalker, A New Hope
- Wind Waker starter costume in swim material
Eventually: Paul from Dune, Faramir, Harry Sullivan