In the reaction #Mg(s) + 2HCl(aq) -> MgCl_2(aq) + H_2(g)#, what role does magnesium play?

In the chemical equation given to you

#"Mg"_ ((s)) + 2"HCl"_ ((aq)) -> "MgCl"_ (2(aq)) + "H"_ (2(g))#

magnesium is a reactant, which is why you see it added to the left of the reaction arrow.

Now, this chemical equation describes a redox reaction. Here magnesium metal is being oxidized to magnesium cations, #"Mg"^(2+)#.

On the other hand, hydrogen is being reduced from hydrogen ions, #"H"^(+)#, to hydrogen gas, #"H"_2#. This can be seen by looking at the oxidation states of the atoms involved in the reaction

#stackrel(color(blue)(0))("Mg") _ ((s)) + 2stackrel(color(blue)(+1))("H") stackrel(color(blue)(-1)) ("Cl")_ ((aq)) -> stackrel(color(blue)(+2))("Mg") stackrel(color(blue)(-1)) ("Cl"_ 2) ""_ ((aq)) + stackrel(color(blue)(0))("H") _(2(g))#

Notice that magnesium goes from on oxidation state of #color(blue)(0)# on the reactants' side to an oxidation state of #color(blue)(+2)# on the products' side, which means that it is indeed being oxidized.

Hydrogen goes from an oxidation state of #color(blue)(+1)# on the reactants' side to an oxidation state of #color(blue)(0)# on the products' side, which means that it is reduced.

Therefore, you can say that magnesium is acting a reducing agent because it reduces hydrogen to hydrogen gas while being oxidized in the process.

Hydrochloric acid, the compound that delivers the hydrogen ions to the reaction, is acting as an oxidation agent because it oxidizes magnesium to magnesium cations while being reduced in the process.