Umeboshi is sour pickled plum and has been eaten for over a thousand years in Japan. Ripened plums are marinaded with salt, sun-dried, and preserved for a couple of months to develop the flavor. Initially, the preserved plums are actually yellow, not the familiar red you typically see Umeboshi. Red perilla leaves (Shiso) are pickled along with the plums to give a nice red color. It tastes salty and sour and goes with rice very well.
Umeboshi have many uses and plays many roles. It can be used whole (with or without a pit) to put in Onigiri rice ball as a filling. Hinomaru Bento (Japanese flag Bento) is just white rice in a bento box with a single Umeboshi placed in the middle of rice. Saltiness of even one Umeboshi can go far to eat a whole bowl of rice without any other dishes. Umeboshi also is said to suppress bacteria, and often is used in Bento Box lunches which may be kept at room temperature for a couple of hours. Umeboshi has a refreshing plum flavor and is good in Okayu porridge when you are not feeling well. It can also be chopped up and added to sauces or other dishes to flavor. A very flexible ingredient and a basic part of Japanese food.