Onigiri is a rice ball which is a most common food when Japanese go for picnic or hiking.
Rice balls have various stuffings. The major stuffings are umeboshi; pickled Japanese plum, grilled salmon, canned tuna flake with mayonnaise, kombu kelp and dried bonito flakes. Then, rice balls are covered with dried nori seaweed.
Onigiri are sold everywhere especially at convenience stores. However, you can buy cheaper at supermarkets.