What does this equation mean?

Zoom out from individual nuclei to the gas that forms whole generations of stars, and the genealogy becomes explicit rather than metaphorical. Astronomers describe the abundance of everything heavier than helium in a star or a cloud of gas with a single number, metallicity, defined through the mass fractions of hydrogen ( X ), helium ( Y ) and “metals” ( Z ): $Z \equiv 1 - X - Y$. A gas cloud’s metallicity is, quite literally, an inherited character: it is the accumulated, largely unerased record of every previous generation of stars that lived, burned and returned processed material to the interstellar medium before that cloud collapsed to form new stars. The first stars, Population III, formed from gas of essentially zero metallicity, fresh from early-universe nucleosynthesis and carrying nothing heavier than trace lithium. Their supernovae seeded the surrounding gas with the first heavy elements, from which Population II stars formed; those stars’ deaths enriched the gas further, and the Sun and its contemporaries, Population I, formed from gas already carrying the metals accumulated across billions of years of prior galactic history [ 9 ] . No individual Population III star has ever been unambiguously identified in observation; the earliest generation is inferred from its enrichment signature in the stars that followed it rather than read directly off a survivor, which is itself an honest limit on the archaeology this section otherwise leans on — descent is easier to trace than the very first ancestor.