It is supposed to be an emoticon. A little face made from letters (or other characters) that can be used to show your mood when you can’t or don’t want to use emoji or pictures. This one especially should be a cute face with the “u” being closed eyes and the “w” being sort of a pouting mouth or an animal mouth. Especially the ones with “w” as a mouth are mostly used by men trying to act cute or girlish. They are almost never used by girls. Other examples would be: O_O or ^_^ . Sometimes you get them with surrounding braces to mark the sides of the face: (^_^) or extra characters like this being a drop of sweat ^_^" which resembles this 😅