In astronomy 'metal' is shorthand for 'all elements except hydrogen and helium' [0]. This is a convenient shorthand because big bang nucleosynthesis [1] produces only minuscule amounts of lithium and even less heavier elements. Nonnegligible presence of elements beyond helium is indication some stars' life cycles completing and their insides being distributed via explosion. So, being metal-poor is an indicator of very young age.
[0] https://en.wikipedia.org/wiki/Metallicity
[1] https://en.wikipedia.org/wiki/Big_Bang_nucleosynthesis