Umeboshi are Japanese plums, related to apricots, which have been salted and dried. The wrinkly red pickles are extremely salty and sour, although sweeter versions exist. Umeboshi serve as a preservative and digestive. They are eaten with all types of traditional meals, and often accompany the rice in boxed lunches (bento). Umeboshi are also one of the most popular fillings for rice balls onigiri.
Japanese Food by Kojiki 