The higgs boson is a fundamental particle in physics. Particle physicists (the guys who run particle colliders and atom smashers) have been searching for it for a long time. Theoretically it should exist because the math that underlies physics says that there should be a particle that allows massless particles (like a photon) to have mass. They call this particle the Higgs Boson.

The reason it's a big deal is that no one has observed it or found experimental evidence of it yet. That's what the new super collider in Europe is supposed to find. If they can find evidence of the Higgs Boson, it will be the last elementary particle to exist and will take scientists one step closer to a grand unified theory in physics...basically dorks get really excited over a single theory to explain the fundamental dynamics of the universe. Now, that doesn't mean we know everything, but it takes us one step closer to figuring out the rules of the universe which lets us understand why things happen.

You can read more here:
http://en.wikipedia.org/wiki/Higgs_boson