When you tape a text without choosing a font the "Font" combo is left empty (you see a blank field). Changing the DeaultFontLabel gives you the possibility to change this behavior. Example:
FCKConfig.DefaultFontLabel = 'Normal Font' ;
You've chosen to label the "Font" combo 'Normal Font'. When you will type the text you will see that the font label is 'Normal Font'. If you choose a font e.g.'Arial' from the combo the label will change as well. If you try to go back to the text written without choosing the font you will see that it is labeled 'Normal Font'. So as you can see the "DefaultFontLabel" determines how the normally typed text will be labeled.
Note that the DefaultFontLabel setting will not define the font to be used by default, just the label to show in the toolbar combo. To set the default font, you must properly define the styles for the "body" element in the CSS file used in the editing area (see EditorAreaCSS).