Umeboshi (梅干し)

(The right picture is from here)

This is umeboshi, it's one of the traditional foods in Japan. It means dried Japanese plums, and it is very sour pickles.

Japanese people can eat a bowl of rice with only one umeboshi because it has a strong flavor: very sour, salty, and a bit sweet. Since it has antibacterial properties, old Japanese people from long ago ate it often and put it in their boxed lunches especially in onigiri (rice balls). Even now umeboshi is a common filling in onigiri. Other than that, there is a type of boxed lunch that has one umeboshi on the rice, and we call it hinomaru bentou (hinomaru is the name of our national flag, and bentou means a boxed lunch) based on the look.

Umeboshi is too sour for children, so when a child is able to eat it, we say things like “Oh, you've become an adult!”