A GUI program consists of a collection of graphical components that are all placed inside one or more windows. The components are contained by the window. A container is an object that can hold other GUI components. Visually, a container is an area of the screen and the objects it contains are shown as smaller areas within it.
In Java terminology, a window (such as the browser window you are looking at) is a container. Buttons, sliders, icons and other GUI components are always put in container.
Do you think that a container can contain another container?