It's Minimizes/Maximizes the window.
Minimize.
Typically (at least in Windows OS), that will at least consist of the Title Bar and (in most cases) the icons to Minimize, Maximize, Restore and/or Close the window.
On a window's Title bar, what does the button with the underscore symbol do?
The close button, the minimize button, the maximize button, the document title and the icon
sometimes
minimise or restore down on some computers XP should say minimise but not 100% sure about it
In MS Publisher, the title bar is the top horizontal bar of the application window that displays the name of the publication you are currently working on. It also contains the minimize, maximize, and close buttons on the right side to manage the window.
just double click on the title bar OR you can click on the maximize button.
It will maximize the window.
Depends on the state
The "Maximize" button, located between the "Close" button and the "Minimize" button.
Title Bar
There should be a line in the right-hand side of your title bar. Click it.