おにぎり onigiri, also called おむすび omusubi, are rice balls. Onigiri are a common item for Japanese lunch boxes, called bento 弁当. You can find onigiri at every bento stand and conbini; Lawsons and FamilyMart usually have 20 different types lined up in the refrigerated section, with a variety of fillings from plain salt to tuna to spam with egg.
For my lunch, I went to a vegetarian-friendly market and picked up an onigiri-bento. It came with 2 onigiri: one wrapped in shiso leaf with nori (seaweed) sprinkles (also known as furikake ふりかけ), and one with shoyu and ginger (and probably some other things); it also came with something shaped like onigiri but with pickled plum/shiso and purple sweet potato (and maybe okara, aka soy leavings, but I am not a hundred percent sure). There was a delicious side of pickled carrot and daikon, mini tomatoes, and potato salad. I also bought a delicious vegan strawberry and poppyseed muffin for dessert. Everything was really tasty, so if you are in Okinawa, be sure to stop by HappyMore Farmers Market in Nakagusuku.