The Vietnamese word "ủ" has a couple of meanings and uses that can be helpful for you to understand as a new learner.
As a verb: "ủ" means "to keep" or "to maintain" something in a certain state, often referring to keeping something warm or protected.
As an adjective: "ủ" can describe a state of being sullen or gloomy. It is often used to express a mood that is not cheerful.
For the verb meaning "to keep":
For the adjective meaning "sullen" or "gloomy":