Tesseract. That's the thing that brought me here, am I correct? That's what they kept mentioning. What is it? How does it work?
And I don't know why it "thinks" and decided I'm more important than either of them, especially Eren. He's the one who they need.
You would know if there were Titans. They live for one sole purpose: to eat humans. The Walls--Maria, Rose and Sina--were built to keep the Titans at bay.