The Higgs Boson is an elusive, hypothesized particle. It is an essential part of the Standard Model of particle physics, but thus far it only exists in theory. This particle gives rise to the Higgs field, which in turn explains why elementary particles have mass. That sounds like an odd concept, but when you come right down to it, what is mass anyway? Think about that for a moment.
Now, scientists have accumulated enough data to detect a moderate signal in favor of the Higgs. At this point, the data is tantalizing, but by no means certain. Many more experiments will have to be conducted in order to say for certain whether the particle has been found. If it has, it will be a triumph of modern physics.
It seems odd that one would spend $10 billion dollars on finding something that may or may not exist, especially with many areas of science starved of funding. On the other hand, is it really possible to put a price tag on discovering the basic foundations of existence itself?