If there is a light Higgs boson, its mass must be between 114 GeV and 158 GeV, according to combined data from two major research projects at Fermilab. The evidence that there is indeed a Higgs boson of that mass, however, is not statistically significant. Many physicists betting that it will be on the light side of that range (140 GeV or less), because the closest thing the data have found to a non-statistically significant sign of a Higgs boson is at a mass of 140 GeV.
(Strictly speaking, a giga-electron volt needs to be adjusted by a factor of the speed of light squared to provide units of mass, but it is customary to omit that factor as a short hand; a proton is about 1 GeV.)
The particle was proposed in 1964 by Peter Higgs to explain why Standard Model particles have mass. If it isn't found then there is something deeply wrong with the theoretical structure of the Standard Model, a possible extension of it called Supersymmetry (SUSY), and an even grander extension of its commonly called String Theory or M-Theory.
The converse is not true. A light Higgs boson does not necessarily imply Supersymmetry or String Theory, although a light Higgs boson would not be inconsistent with them. The key discovery necessary to validate either of those theories would be the discovery of the LSP, the lightest supersymmetric particle, hypothesizes to have a mass of around 500 GeV or so. This would be within the range of the Large Hadron Collider's ability to detect, but beyond what could be found at Fermilab.
While many physicists are still giddy with the possibility of new physics as the window of possible masses (and hence the range of possible theoretical models and time frame for possible discovery of the Higgs) narrows, I'm skeptical. It seems at least as likely to me that there is nothing in the narrow remaining range of masses.
The fact that physicists have searched so long without finding anything makes the assumption that the one undetected predicted particle is in the mass range that can't be detected seem like wishful thinking. Experiments have produced many unpredicted phenomena in the three and a half decades since 1964, in addition to the few predicted but undiscovered ones they have revealed.