Umeboshi

Umeboshi are pickled (brined) ume fruits common in Japan. The word umeboshi is often translated into English as 'salted Japanese plums', 'Japanese plums' or 'preserved plums'. Ume (Prunus mume) is a species of fruit-bearing tree in the genus Prunus, which is often called a "plum", but is actually more closely related to the apricot. Pickled ume which are not dried are called umezuke (梅漬け). Umeboshi are a popular kind of Japanese tsukemono ('pickled thing'; preserved or fermented) and are extremely sour and salty. Sweet umeboshi made with honey also exist. They are usually served as a side dish for rice or eaten on rice balls (often without removing the pit) for breakfast and lunch. They are occasionally served boiled or seasoned for dinner. Umeboshi are usually round and vary from smooth to very wrinkled. Usually they taste salty, and are extremely sour due to high citric acid content, but sweeter versions exist as well.

Source: Wikipedia