What does this equation mean?

The theorem is where the story becomes genuinely subtle, because the Standard Model’s electroweak sector does not contain any observed massless scalar of the kind Goldstone’s argument predicts, and the reason is itself a discovery rather than a loophole. Englert and Brout, and independently Higgs, showed in 1964 that when the broken symmetry is a local gauge symmetry rather than a global one, the story changes at the last step [ 4 , 3 ] . The gauge bosons associated with the broken generators absorb the would-be Goldstone modes as their own longitudinal polarization states, and in doing so become massive. What would have been a massless, physically propagating Goldstone boson is not observed as a separate particle because it has been consumed as a component of a gauge boson that previously, being massless like the photon, had no longitudinal mode to have. Three of the electroweak sector’s four generators are broken this way, giving mass to the $W^+$ , $W^-$ , and $Z^0$ ; the fourth, corresponding to the unbroken electromagnetic U(1) , is not, which is why the photon remains massless. Weinberg’s 1967 paper assembled this mechanism into the specific gauge theory of leptons that, combined with the analogous quark sector, became the electroweak part of the Standard Model tested at the LHC decades later [ 6 ] .