Best ask her. It's a thin board of some sort, but I can't be sure exactly.

It’s called art or matte board. Mine is white. That background is a combo of parchment colored acrylic paint, then this stencil on top using a black ink pad, then a bit more paint so the stencil looks faded.

I have a bunch of art boards with paint and stencils on them left over from other projects. For that #puffpuffpaint session, using the art boards was a fast way to start because the background already existed. #artstr #mixedmedia #🖼️