It is polyester bolting cloth.
About the polyester printing mesh:
Polyester was introduced to the screen printing market over 40years ago and quickly replaced traditional silk as the fabric of choice among screen printers all over the world. It is a material that is uniquely suitable for screen printing because of its tensile strength, elastic memory and resistance to chemicals, abrasion heat and moisture. Polyester screen is a kind of synthetic fabric constructed similar to silk, yet with better uniformity. Compared to silk, polyester screen cloth offers extreme durability, yet has a lower price.