Traditionally the ume are dried and then preserved in a salt brine with purple perilla (shiso) leaves that add a bright red color. These days, some brands use red dye in place of the leaves as well as less salt, which can prompt the use of artificial preservatives. The Eden brand, which is widely available in the US, uses the traditional method. The ume fruits are pressed and the liquid that comes out of them is bottled up for use as a condiment.
Umeboshi vinegar is used widely in Japanese cooking as a seasoning. It's tossed with steamed vegetables and sprinkled over sautéed greens and rice. In the US, it first became popular outside of Japanese communities in the 1970's when people began exploring macrobiotic and Japanese cooking. The vinegar is super salty, with a sour, fruity flavor. It is quite unique and can be creatively applied to vegetarian and vegan dishes where a sea-like or fish flavor is desired. Some people use it in place of fish sauce, although its flavor is fruitier and brighter to my palate. Be careful to adjust any salt in your dish, as often the addition of umeboshi vinegar will be enough.
(Image: Lynn Chen/The Actor's Diet)