It is a container for storing water, which is used for adding more water to chagama (iron pot used in the tea ceremony) and rinsing tea bowls and chasen (tea whisk) with water, when making tea in the tea ceremony.
茶道で、点茶の際、茶釜に水を足したり、水で茶碗や茶筅をすすぐため、蓄えておく水器。