The following article explains the page bleed and safe area guides in Adobe Lightroom. This example uses Lightroom 4 but the guides are similar in subsequent versions.
This is what you'll see in Lightroom when using the Book module. Ignore the Photo Cells and Filler Text options for now.
Show Guides: Page Bleed and Text Safe Area
Here's a page in a Lightroom book with the guides turned on. Notice the opaque grey shaded area on the very outer edge (the Page Bleed) and the inner thin grey line (the Text Safe Area). The black area is just the background around the page in Lightroom.
Pro tip: you can change the background color (the color that displays around the page) by right-clicking on the background and choosing another color.
And here's the page with the guides turned off (by deselecting Show Guides).
The page bleed guide indicates where your full bleed image must reach to in order to completely fill the printed page.
Here I deselected Photo Cells and left the page blank to more easily show the opaque grey area that makes up the page bleed.
And here's the same page with an image placed in it.
Design your bleed correctly
If you want your image to completely fill the page (a "full bleed" image) then you must ensure that the edges of your image reach all the way to the outer edges of the page bleed.
Here's an incorrectly placed image. It's incorrect because the image's edge doesn't reach all the way to the bleed edge.
Here's a close up of the incorrectly-positioned image. The photo's edge doesn't reach the outer bleed edge in Lightroom. The printed book would likely have a thin white edge at the top.
Here's a close up of a correctly positioned full bleed image in relation to the page bleed. Notice how the image edge goes all the way to the page edge.
Text Safe Area
Think of this line as just the "safe area" because it applies to text and any other important content.
- Do not place any important content (text, faces, etc.) on or over the Text Safe Area line.
- Any content on/over the trim line might be trimmed off or lost in the gutter.
Good layout example: important content does not cross the grey trim line
Here's a close up of the earlier image. Notice how the cat's ear does not cross the safe area line.
Bad layout example: important content crosses the grey trim line
Here I incorrectly positioned the image so that the cat's ear crosses the safe line. It could be trimmed off in the final printed, trimmed and bound book.