This is called a "sweep." It is a piece of cloth hung that sweeps forward to the front of the staging area - so there is no seam or line at the back. In this case, it is painted canvas - painted quite artfully to shade a little lighter in the middle and darker at the edges. You could easily paint your own.
I have a series of "stages" - which are plexiglass, bent into a small, raised table.This is the clear one.
This one is mirrored - shiny silver. It is not as reflective as a mirror per se, reflections are softer and less focused.
This is an old slate roofing tile (part of one, anyway.) I salvaged it many, many years ago.
And this is the sort of picture you get. This is the canvas sweep.
This is on the clear stage. This give you reflections, but also the ambient colour of your background.
This is on the mirrored stage.
This is on the black stage.
This is also on the illuminated panel - but the camera was set to "Hi-Key" - a setting that over-exposes the image and reduces the contrast. The back ground is much closer to white. Not white, but closer.
Try your local dollar store and look for serving trays in plain white or black to serve as a staging area for your photos.
If you have any more photography questions, please feel free to leave them in the comments section!