while i was looking for a cute image of North to post in the chat room i came across this image

I remember in Season 2 Episode 8 - Chinatsu Unrivalled (i think) Toshinō Kyōko gets everyone to help her with her doujin work by doing some shading. from what i remember they place the darker sheet over top of the outlined doujin pages.

So what exactly is this sheet? what is it used for? is it something Doujin Artists use to supplement what a professional would use or is it used by Mangaka's aswell?