UwU is a text based emoticon, similar to OwO.
It is used to express feelings of happiness and smugness, while making the online persona of whoever types it, cute.
The emoticon is often associated with being used by old men, trying to appeal to little girls, by using their way of expression.
UwU is said to be originating from chatrooms, although it is not exactly clear. The cute face was already present in a Yu-Gi-Oh fanfiction in 2005.
It is often found in chatrooms, about manga and anime, or Japanese culture, in general, since this group was the first to use the expression, at all.
Since the 2010’s UwU had spread into common use, from sites like 4chan, to Tumblr.
UwU is widely embraced by sceptics, in an ironical way, but the very same people seek any opportunity, to pick on those who use the emoticon in a serious manner.