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 – like 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.
You will always need a bleed on your cover. If your interior pages include printed areas that extend to the edge of the page you will need to add a bleed of 0.125" (3.175mm) to the top, bottom, and outside margins. You don't need to add a bleed to the inside margin.
So, for example, if you chose a 6" x 9" trim size, you would need to adjust this to 6.125" x 9.25".
You would then need to ensure that the relevant printed areas extend right to the edge of the newly expanded page, but you would also need to ensure that no text ever appears within the bleed (i.e. all text must be at least 0.125" away from the top, bottom, and outside edge of the page). This is because the page may be trimmed anywhere within this area, and so any text within it risks being partially lost.