An oxidising agent is the one which oxidises other substances (by removal of hydrogen or addition of oxygen) and itself gets reduced (addition of hydrogen or removal of oxygen).
CuSO4+Zn→ZnSO4+Cu
In the above reaction, copper sulphate (CuSO4) acts as an oxidising agent as it oxidises zinc (Zn) to form zinc sulphate (ZnSO4) and copper (Cu).