This accessory, which is known as a horoku, or sometimes a hoji-ki, is used to make hoji-cha at home.
All you have to do is put 3–5 grams (1-1.5 tsp) of Japanese green tea in the horoku, and then swirl the tea in it quickly over a low or medium flame on a gas stove.
Stop the roasting as soon as the tea has the desired colour. You can roast hoji-cha to your own taste: very little (recommended for high-quality teas) or dark (recommended for lower-quality teas).
We recommend you use traditionally steamed teas with large leaves, or, better yet, a kuki-cha.
There is a demonstration in pictures here.