Umeboshi - which translates as dried ume - are a Japanese salt pickled plum which are intensely salty and sour. The first written records of umeboshi appear as early as the year 200, prizing ume-su (ume vinegar), the sour-salty liquid byproduct of the umeboshi-making process, for use as an antiseptic on wounds.
Umeboshi are used as a condiment rather than a pickle. They are usually served with rice for breakfast or lunch and in small quantities to flavour dashi sauces. It is also a common ingredient in onigiri, rice balls wrapped in nori seaweed.
Traditionally made umeboshi can keep for years, even decades, and if they do go bad (they turn black) it's thought to be a very bad omen.