Galactic chemical evolution witnesses the enrichment of the interstellar medium with elements heavier than H, He, and Li that originate from the Big Bang. These heavier elements can be traced via the surface compositions of low-mass stars of various ages, which have remained unaltered since their formation and therefore measure the composition in the interstellar medium at the time of their birth. Thus, the metallicity [Fe/H] is a measure of the enrichment with nucleosynthesis products and indirectly of the ongoing duration of galactic evolution. For very early times, when the interstellar medium was essentially pristine, this interpretation might be wrong and perhaps we see the ejecta of individual supernovae where the amount of H with which these ejecta mix is dependent on the energy of the explosion and the mass of the stellar progenitor. Certain effects are qualitatively well understood, i.e. the early ratios of alpha elements (O, Ne, Mg, Si, S, Ar, Ca, Ti) to Fe, which represent typical values from Type-II supernova explosions that originate from rapidly evolving massive stars. On the other hand, Type-Ia supernovae, which are responsible for the majority of Fe-group elements and are the products of binary evolution of lower-mass stars, later emit their ejecta and reduce the alpha/Fe ratio. In addition to being a measure of time, the metallicity [Fe/H] also enters stellar nucleosynthesis in two other ways. (i) Some nucleosynthesis processes are of secondary nature, e.g. the s-process, requiring initial Fe in stellar He-burning. (ii) Other processes are of primary nature, e.g. the production of Fe-group elements in both types of supernovae.