Jonno,

They are trying to re-create the environment a billionth of a second after the Big Bang to look for the birth of the Universe and among other things trying to find the Higgs Particle, by some people referred to as the God Particle. It is the very fabric of space and all matter it contains. The stuff atom parts are made of. That is the hypothesis at least. This experiment is trying to establish evidence for it.

Also see http://www.youtube.com/watch?v=fPxYdObyJ2A for a nice explanation.