As any student who studied a bit of chemistry knows that metals in the periodic table are various, after all the periodic table is usually taught to students as including group 1 and group 2 corresponding to alkali metals and alkaline earth metals, respectively.
Since the universe is mostly made up of hydrogen (74%) and helium (24%) , astronomers often refer to all the other elements we know of as “metals”. What we know about the “metallicity” is that it is tightly related to the age of the star. In other words, astronomers categorize stars into populations (I, II, III) based on their metallicities. Initially a star is born with mainly hydrogen and some helium (i.e; low metallicity). As the star burns the hydrogen, the helium gets more abundant and as the helium and hydrogen are consumed new elements (metals) are formed. So logically, young stars have low metallicities (these belong to population II) and old stars have high metallicities (population I)! Population III stars are those that have no metallicities and are yet to be discovered.
The metallicity is denoted by the letter Z as well as [Fe/He], the latter of which is read “the ratio of iron to helium” (i.e; abundance of Fe / abundance of He). For example, one of the brightest stars in our sky is Sirius, a main sequence star. It has a [Fe/He] = 0.5 . It should be noted here that Z and [Fe/He] are calculated differently so they have different but correlated values, where [Fe/He] is logarithmic in that [Fe/He] = log(abundance of Fe / abundance of He)!
Below is an example of a plot of the logarithmic (base 10) abundance versus the atomic number, Z, of the elements, the latter not to be confused with metallicity, in the solar system!
 of all baryonic matter!
 The Abundance Patterns of Sirius and Vega [H. M. Qiu et al. 2001] doi:10.1086/319000