woruber: has this word been discussed before?
Worüber was covered before, in the "Questions" lesson.
My first time seeing it!
i used "I do not know what is she talking about", duolingo rejected it. Is that my mistake?
Yes. You had "is" in the wrong spot. It should look like this:
"I do not know what she IS talking about"
What is the role of the word "weiß" in this sentenc?
It is the verb. "Ich weiß" means "I know".