Onigiri literally means taking hold of (something) with your hands. Onigiri is made by cupping hot, freshly cooked rice in your hands and pressing down on it so that it is firm on the outside and soft inside. Onigiri usually have something—a treat— inside, but they are about the fine flavor of the rice.
Onigiri is a popular way to take Japanese food on the go. Think of it as a Japanese sandwich! It’s a hand pressed ball of rice, usually filled with a salty or vinegared treat, and often wrapped with a piece of nori or coated with sesame seeds. Popular fillings include umeboshi, tsukemono, smoked salmon, salted cod roe, katsubushi, or any thing you like that is not too greasy, juicy, or likely to spoil easily. You want something with a strong flavor to balance the blandness of the rice. Ham? Avocado? And now apparently popular: canned tuna and mayo! Onigiri can even be eaten without any filling at all!