An electroscope is a device that can be used to test for the presence of charge, or that can be charged. An electroscope is made from conducting material (generally metal). Charge is free to flow on a conductor, and if you put charge at a particular point it will distribute itself over the surface of the conductor.
An electroscope generally has a rotating arm indicating the presence of a net charge. On the electroscope we'll use, when the electroscope is uncharged the arm is usually vertical, and when the electroscope is charged the arm moves away from vertical. This is true as long as there are no charged objects nearby - in a few minutes we'll discuss what happens when there are charged objects nearby.