What does this equation mean?

Albert Einstein’s paper “On a Heuristic Point of View Concerning the Production and Transformation of Light” reached the editors of Annalen der Physik on 18 March 1905 and was published that June, one of four papers from what is now called his annus mirabilis [ 1 ] . Its actual claim is narrower than the “light is made of particles” shorthand it gets today. Einstein proposed, as a heuristic — his own word, chosen because he was not yet willing to overturn the wave theory that already explained diffraction and interference — that “the energy of a light wave emitted from a point source is not spread continuously over ever larger volumes, but consists of a finite number of energy quanta that are spatially localized at points of space, move without dividing and are absorbed or generated only as a whole” [ 1 ] . In his own notation, the energy of one such quantum was the constant ratio Rβ⁄N — built from the gas constant, a term from Wien’s radiation law, and Avogadro’s number — multiplied by the frequency ν. That ratio is the quantity every physics student now writes as Planck’s constant, so the relation reads: $E = h\nu$. energy proportional to frequency, with nothing smaller possible in between. That single relation is what let Einstein correctly derive the photoelectric effect’s otherwise strange behavior: raising a light’s intensity changes only how many electrons a surface ejects, never their individual energy, while raising its frequency raises that energy directly. It is also, specifically, the achievement the Swedish Academy cited when the 1921 physics prize — reserved a year and awarded in 1922 — went to Einstein “for his services to Theoretical Physics, and especially for his discovery of the law of the photoelectric effect” [ 2 ] . Not for relativity, which the committee still treated as unconfirmed. Einstein’s Nobel is for the light quantum, full stop.