The Higgs boson is a fundamental particle tied to the Higgs field, an energy field that fills all of space. Particles that interact strongly with this field, like the top quark, are heavy, and particles that barely interact with it, like the photon, are massless.
The Higgs boson itself is what you get if you excite that field enough, the same way a ripple is what you get if you disturb a pond. It was predicted in 1964 by Peter Higgs and others, and it took nearly fifty years and the construction of the Large Hadron Collider to actually detect one.
Its discovery in July 2012 at CERN completed the Standard Model’s roster of particles and confirmed the mechanism by which most known particles get their mass at all, one of the last major open pieces of twentieth century physics finally closed.