Comment by diedyesterday

Fundamentally, "mass" is nothing other than "confined energy". Whenever you have a mechanism for confining energy (Higgs mechanism; or a force which binds and creates a bound state) the combined package has inertial "mass"; This means: It cannot move at speed of light (c), it can change its speed and it takes energy to change its speed (a "massless" particles does none of this).

The famous thought-experiment in the regard is Einstein's "photons-box": If you could confine a bunch of massless photons (which only have kinetic energy and momentum) inside a (massless) box made out of mirrors, (he argues) the combined package would have "mass", even though the constituents do not (and the emergent "mass" equals E=m c^2 !). In other words, "mass" is an emergent property of the confined ensemble. All of the forces (especially the strong force), create bound-states which are massive and are the exact analogues of this "photon-box".

So the mass of a proton (mostly) comes from the kinetic energy of its confined (by the strong force) constituents (the quarks and gluons).