The metal rod of an electroscope is used to conduct electricity, while the rubber stopper serves as an insulator. When you touch a charged object to the metal rod, the charge is transferred to the rod and the electroscope becomes charged. If you were to touch the charged object to the rubber stopper, the charge would not be transferred because rubber is an insulator. So, you must touch the metal rod to charge an electroscope and not the rubber stopper.