Lets say that you have created a table in the editor and you've set the "Border size" to "0". If the "ShowBorders" option is enabled (set to 'true') the editor will show the borders of the table so you can easily access the cells. Of course the table will still have the border size set to 0. If you disable this option when you create a table with the "Border size" set to "0" you won't be able to see the table in WYSIWYG mode and you will have to access it from the "Source Code" view.
FCKConfig.ShowBorders = true ;