When you tape a text without choosing its style the "Style" combo is left empty (you see a blank field). Changing the DeaultStyleLabel gives you the possibility to change this behavior. Example:
FCKConfig.DefaultStyleLabel = 'Standard Style' ;
You've chosen to label the "Style" combo 'Standard Style'. When you will type the text you will see that the style label is 'Standard Style'. If you choose a style e.g.'Computer code' from the combo the label will change as well. If you try to go back to the text written without any style you will see that it is labeled 'Standard Style'. So as you can see the "DefaultStyleLabel" determines how the normally typed text will be labeled.