#ffffff is the hexadecimal color code for white, a color coding system that is typically used for websites. (Also RGB: 255, 255, 255) What they are saying is that the background color of all your paragraphs is set to the color white, instead of transparent (or, no color set at all). An ebook is essentially a self-contained website, and uses the same methods for coding the layout. One of those methods is Cascading Style Sheets (CSS) where things like the background colors are set. Which means that the formatting software that you used to create the ebook has a background color setting of #ffffff for the paragraphs.
How that got in there is something you either need to know how to set in the ebook-producing software, or be able to edit in the CSS.