The Product Description element outputs the product description content you add to your product in admin edit screen. This is the same content you see in the product tab - description.
If you do not want to use the default product tabs and instead add the Fusion Builder tabs to your product, you can simply use this element's shortcode to output the product description to your tabs content. You can then customize it the way you want.
Here's a quick view of how you can achieve better results with Fusion Builder tabs and the product description element -