Not v. high confidence level as I started out thinking this might be a sort of finish, like a resin coating, but then I found this, about bags no less:

What is Underglass?
Underglass is a handbag design feature where we sew a thick plastic layer over the printed fabric. This creates a small air pocket between the plastic and the fabric.

here's the link: https://www.ginaalexander.com/questions.htm#underglass

also found "underglass" effect referring to a resin coating though...