The bamboo mat used for rolling sushi is called a makisu. Its name comes from Japanese, with maki referring to rolling and su meaning a mat or screen.
A makisu is traditionally made from thin bamboo strips woven together with cotton string. Sushi makers use it to shape makizushi, including familiar rolls such as futomaki and hosomaki. It can also help shape tamagoyaki, the Japanese rolled omelet, and press liquid from certain foods.
Most makisu are roughly 25 by 25 centimeters, though larger and smaller versions exist. For inside-out rolls, called uramaki, cooks sometimes cover the mat with plastic film so rice does not stick between the bamboo strips.
Makisu is often confused with nori, which is the seaweed wrapper, or with hangiri, the wooden tub used for cooling sushi rice. The mat shapes the roll; it is not an ingredient.