calligraphy mat, in Chinese it is called Shufa dianbu (书法垫布, Shūfǎ diàn bù). It is used to protect the surface where you place the paper so it cannot be damaged.
The mat is made of thin felt and it is hard to wrinkle.
In addition, it has white lines so the writer does not actually get out of the paper when practicing. It has 6 squares in one side and 4 in the other one.