Ume (梅) or umeboshi (梅干し) is Japanese salted plums or Japanese pickled plums. It is a popular kind of Japanese pickles (Tsukemono) and they are extremely sour and salty. We usually serve umeboshi with rice or rice balls (onigiri).
Among all kinds of umeboshi, Nanko Plums (南高梅 なんこううめ) from Wakayama prefecture (they are white plum) are considered as the highest quality.