Texon reinforcement (stiffener) material.
Available thicknesses: 0.6mm and 0.8 mm in black color

Set of 5 Sheets of 20 x 30 cm in size

It's a paper-like stiffener.
It is called Texon, Bon Tex or Web-Tex depending on the manufacturer.
Texon is used to line and reinforce leather projects (backs and bottoms of bags, strap ends, etc) or give thinner leather the right consistency for some projects.
Natural fiber construction material (wood pulp, cellulose) bonded with elastomers. It's lightweight, free from any hazardous chemicals, and dimensionally stable.