These red foods is called Umeboshi, a fruit of ume, Japanese apricot. They are pickled in salt with red perilla and dried in the sun. Those who taste them for the first time may find them surprisingly sour. They are used in various dished as well as being served with rice. They are often included in boxed lunches as a garnish for rice and as a filling for rice balls, called Onigiri.
Some people put it to Shochu, a traditional distilled spirit, adding hot water. The combination is also nice and I love it.