For an excellent introduction to the Higgs Boson, see
According to the Englert-Brout-Higgs mechanism, the property that we measure as the ‘mass’ of a particle is the result of a constant interaction with a field that permeates the Universe like a sort of “ether”. The existence of this Englert-Brout-Higgs field is definitively proven by the discovery of the corresponding quantum particle – the Higgs boson.
It should not be thought that the Englert-Brout-Higgs field is responsible for all the mass in the Universe. Your interaction with the field actually contributes less than 1 kg to your mass. The remainder comes mainly from the strong force binding quarks inside nucleons, with a tiny contribution from the electromagnetic force that reigns over the atomic and molecular scales.
Higgs bosons are quantum fluctuations in the Englert-Brout-Higgs field that are visible experimentally only when energy is “injected” into the field. Concentrating the right amount of energy in proton-proton collisions at the LHC excites the Englert-Brout-Higgs field, which resonates at a precise energy corresponding to the mass of the Higgs boson. The Higgs boson appears momentarily before decaying into other particles that the LHC experiments can measure. Some theories predict the existence of multiple Higgs bosons.