This is a bean paste made from red adzuki beans that are boiled, drained, then puréed.
It’s called “sweet”, not because sugar has been stirred in, but unlike some other bean pastes such as Miso, it’s not fermented to make it sour. The “sweet” here means not tangy. That being said, some versions actually do add a sweetener to it for use in desserts.
It can be used to make Sesame Seed Balls, as a filling for pancakes and dumplings, etc.
To make your own: simmer 2/3 cup dried red adzuki beans in water until tender, about 1 1/2 hours. Drain, and purée either in a food blender or processor. Meanwhile, heat 1/4 cup of oil on medium-low in a frying pan. Turn the bean mixture into the frying pan and fry for a few minutes — the idea is not that you are so much frying them, as drying the water out so that they will form a good paste. Cool and use. You can store in a sealed container in the fridge for up to 1 week.
To make the actual “sweet” version, add 1/2 cup of sugar while the beans in the food processor.
Called “gochujang” in Korean.