If charges are bound on an insulator then how

A charged glass rod has some excess charge on it which is localized in certain area as electrons are no free to move. When this same region is brought in contact with a metal disc, this excess charge can now flow over from the rod into the disc and that is what happens in an electroscope also. So, please note that the intrinsic electrons of the insulator are not free to move, however, any excess charge that has been put on the insulator can flow over from one body to another even thought they might not be able to move over entire glass rod.