When you type text without choosing the font format the "Format" combo is left empty (you see a blank field). Changing the DeaultFontFormatLabel gives you the possibility to change this behavior. Example:
FCKConfig.DefaultFontFormatLabel = 'Standard format' ;
You've chosen to label the "Format" combo 'Standard format'. When you type the text you will see that the format label is 'Standard format'. If you choose the format to 'Heading 1' from the combo, for example, the label will change as well. If you try to go back to the text written without changing the format you will see that it is labeled 'Standard format'. So as you can see the "DefaultFontFormatLabel" defines how the normally typed text will be labeled.