gtkmm has various widgets that can be visually adjusted using the mouse or the keyboard, such as the Range widgets (described in the Range Widgets section). There are also a few widgets that display some adjustable part of a larger area, such as the Viewport widget. These widgets have Gtk::Adjustment objects that express this common part of their API.
So that applications can react to changes, for instance when a user moves a scrollbar, Gtk::Adjustment has a value_changed signal. You can then use the get_value() method to discover the new value.