the Higgs field
Picture all of empty space as quietly filled with an invisible substance, present in every cubic centimeter of the universe — under your chair, between the stars, inside the atoms of your hand. You cannot see, smell, or feel it directly, yet particles moving through it are slowed and given their heft, the way a person wading through water moves differently than through open air. That all-pervading something is the Higgs field. It is not made of particles sitting somewhere; it is a property of space itself, switched on everywhere.
In modern physics, every particle is really a ripple in a corresponding field that fills space — the electron is a ripple in the electron field, light is a ripple in the electromagnetic field. The Higgs field is one more of these fields, with one decisive difference: its natural lowest-energy state is not zero. Most fields are 'off' (value zero) when nothing is happening, but the Higgs field sits at a nonzero value even in completely empty space. Particles such as the electron, the quarks, and the W and Z bosons interact with this ever-present background, and that constant interaction is what we measure as their mass. The more strongly a particle couples to the field, the harder it is to accelerate, and the more mass it has. A particle that ignores the field entirely, like the photon, stays massless and flies at light speed.
The Higgs field was proposed in 1964 by Robert Brout and François Englert, and independently by Peter Higgs (with related work by Guralnik, Hagen, and Kibble), to solve the mass problem. For decades it was a brilliant hypothesis with no direct proof. The decisive test came in 2012, when experiments at the Large Hadron Collider produced the Higgs boson — a particle that is, in effect, a small disturbance or ripple of the Higgs field — confirming the field is real. An honest clarification: the Higgs field does not give mass to everything. It supplies the mass of fundamental particles, but the bulk of the mass of protons, neutrons, and therefore of you, comes from the energy of the strong force, not the Higgs.
A common analogy: a famous person crossing a crowded room gets surrounded by a clump of fans and so moves slowly, while an unknown person walks straight across. The crowd is the Higgs field; the celebrity is a heavy particle that couples strongly; the unknown person is a massless one like the photon. The picture is imperfect but captures the core idea — mass as resistance from an ever-present field.
The 'celebrity in a crowd' picture: mass as drag from a field that fills space.
The Higgs field is not the same thing as the Higgs boson: the field is the ever-present background, and the boson is a localized ripple in it that we can briefly create and detect.