Umeboshi (梅干, pickled Japanese plum) is a kind of pickled food. It's one of the Japanese traditional foods and it has been popular in Japan. It is red, round, and a little bit seamed, and its taste is very sour. Japanese people usually eat it with rice, resulting in what looks like the national flag of Japan, with the circular bright red center on a white field. The Japanese also put it into rice balls. It is said that umeboshi is good for health, and sometimes umeboshi is used as a cold remedy. A pickled apricot never rots, so you can eat those which were made 100 years ago.
See also
wikibooks:Cookbook:Umeboshi