* Metal is a conductor: Metal allows electrons to flow freely through it. When you touch a charged object to the metal rod, electrons can move from the charged object to the rod (or vice versa), depending on the charges involved. This transfer of charge causes the leaves of the electroscope to diverge.
* Rubber is an insulator: Rubber does not allow electrons to flow easily through it. If you touch a charged object to the rubber stopper, the electrons will not be able to move to the metal rod, and the leaves of the electroscope will not be affected.
In summary, touching the metal rod allows for the transfer of charge and demonstrates the electroscope's functioning. Touching the rubber stopper prevents this transfer and renders the electroscope useless for detecting charges.
