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.
Finding the Higgs boson took two independent, competing experiments at the Large Hadron Collider, called ATLAS and CMS, each sifting through the debris of hundreds of trillions of proton collisions looking for the same faint signal. Because the Higgs boson itself decays almost instantly, physicists never see it directly, they infer its existence by spotting an unusual excess of particles, like pairs of photons, appearing at a specific combined energy that matches a particle roughly 125 times heavier than a proton. Both teams announced the discovery on the same day in July 2012, a rare instance of two rival collaborations independently confirming the exact same result within hours of each other, which is part of why the announcement carried so much weight.
It is worth noting the Higgs mechanism only accounts for a fraction of the mass in ordinary matter. Protons and neutrons get most of their mass not from the Higgs field directly but from the binding energy of the strong force holding their component quarks together, meaning the Higgs boson’s discovery closed one major gap in physics without being the whole story of where mass comes from.