If the relative activities of two metals are known which metal is more easily oxidized

The metal that has a greater reactivity is more easily oxidized.

Explanation:

Oxidation is when the elements lose electrons and increase their oxidation state.

The metals tend to react by losing electrons and form the corresponding cation.

For expample, sodium (an alkalyne metal) loses one elecron and form the cation Na¹⁺ , then this cation combine with an anion and form compounds like NaCl, NaOH. The same do the other alkalyne metals.

Magnesium (an alkalyne earth metal) loses two electrons and form the cation Mg²⁺, then it combines with some anions to form compounds, like MgSO₄, Mg(OH)₂.

So, the easier the metal gets oxidized the greater its reactivity.