Umeboshi is a traditional Japanese condiment made from salted and pickled ume fruits, which are a type of sour plum. The process involves drying and curing the plums with salt and then fermenting them. Umeboshi is known for its strong and salty flavor and is often used as a topping or accompaniment in Japanese cuisine, such as on top of rice or in rice balls (onigiri). It's also believed to have various health benefits and is considered a natural remedy for digestive issues in some cultures.