UwU is a "emotion" told mostly by weebs and internet people that are attacked after saying it. UwU is shown for cutenes, but people (including myself) hate it when they hear it.
"UwU" is often placed at the end of a sentence and also used when you are either trying to be desperate, give off happiness and affection, trying to look cute or implying that you want to get fucked.
It's usually used by cringe little kids, so I don't advise you to say this personally.
uwu is an emoticon type face similar to :3 and twt. It's used when one is feeling happiness or joy, it can also be described as a feeling of when you see something cute. However, this face has gotten quiet popular amongst young teens, as they have using this emoji ironically.