Our modern understanding of reality is based on some fundamental concepts: that the world around us is tangible, that the theory of relativity holds, that cause and effect works as we'd expect, and that humans have free will. But take quantum theory at face value, and it turns out the four can't co-exist together.
That, that seems like it could be a problem. This video, put together by New Scientist as part of YouTube's Geek Week, attempts to explain -- which I certainly can't. So you better watch it. [YouTube]