http://www.slate.com/articles/health...her_we_re.html
http://www.huffingtonpost.com/2012/1...n_2339109.html
Basically the theory goes that eventually humanity will develop computers powerful enough to simulate entire universes, and that considering this is possible and the vastness of space and time, it is incredibly likely that we are in fact in a universe that is a computer simulation of some other species.
Atheism on the other hand is a strict denial of any kind of all powerful creators or supernatural powers, both of which would exist if we live in a simulated universe.