The temperature of sushi rice is very important. If it is too warm it won't be in harmony with the neta (which should typically be at room temperature, although this varies), and if it is too cool it will taste too salty and lose its vinegary flavor. Cold sushi rice tastes horrible and should never be served. Ideally, shari should be served at body temperature.
A small wooden ohitsu container (such as the one picture above) made from Japanese cypress is used to control the temperature of the rice. Not only does it keep rice at a perfect temperature for several hours, but it also absorbs excess moisture and has antibiotic properties that help preserve the rice.
The ohitsu is typically covered by a cloth and wood lid. If further insulation or temperature control is needed, the ohitsu can be placed inside a woven straw container called a warabitsu.