Popstring class is the base class for creating so called popup objects. Usually, a popup consists of a string or text gadget, followed by a little button. Pressing this button brings up a little window with a listview and lets the user choose an entry with the mouse.
Popstring class features the basic functions for creating such objects. Given a string object and a button object, it places them horizontally and sets up some notification. Whenever the popup button is pressed, a hook will be called which itself should open and prepare the popup window.
The string and the button object are not created by popstring class, they have to be supplied as attributes during object creation time. This makes popstring class very flexible, one could e.g. use a text object instead of a string or a popup button with some text in it.
However, creating simple popups with popstring class would be too much overhead. Instead of using it directly, you should have a look at one of its subclasses. They offer a more specialized set of popups and are a lot easier to use.