
Clicking the button brings up a standard Windows "Open File" dialog
where you can locate and open the file you want to work with. If the
map currently visible in the GUI has been modified, you will be given
the chance to save it before you load the new map.
The "Save" button provides the complementary function, that of saving
the map currently being edited in the GUI to disk. It looks like this:

Clicking the button brings up a standard Windows "Save File" dialog
that allows you to name the map or to overwrite an existing map.