What is meant by "bleed"?
When books are printed, the content is printed on large sheets of paper which are then trimmed down to size by machines. When the pages are cut, there is a tiny margin of error. This normally doesn't matter, because there is normally a sizable margin of blank white space between the text and the edge of the page, but in some cases – particularly book covers – the printed content is intended to go right up to the edge of the page. To ensure that there is no unsightly sliver of white between the edge of the printed material and the edge of the page, the image (or shaded area, etc.) has to be set to spill over the edge of the page by a prescribed amount. This is called the bleed.