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.
Guides turned on
Here's a book page with the guides turned on. Notice the opaque grey shaded area on the very outer edge (the Page Bleed) and the inner, thin black line (the Text Safe Area).
Guides turned off
And here's the page with the guides turned off (by deselecting Show Guides).
Page bleed guide
The page bleed guide indicates where your full bleed image must reach to in order to completely fill the printed page. Here we 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.
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.
Here's a close up of the incorrectly-positioned image. The printed book would likely have a hairline 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
This line should really just be called just the "safe area" because it doesn't apply only to text.
- 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 we 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.