Most printers cannot print to the edge of the paper. To pull the paper through the printer or printing press, space is needed for the gripper to grab hold of the edge of the paper and pull it through. If one attempted to put ink along this space it would merely smear all over the place in a cacophony of blurred color.
Instead what is done is that the publication is printed on paper larger than the final size and then trimmed down. This seems simple enough, but there is a catch, particularly on large-scale printing presses which involve a lot of moving parts: vibrations.
Vibrations are a problem that haunts the printing processes. If you have ever picked up a newspaper where the layers of color do not seem to quite line up, chances are good that it is because the equipment has had enough vibration as it has been run that the color separations are no longer lining up correctly. For big jobs, the press will periodically need to stop and make adjustments to counteract the vibrations.
If one sets the color to be printed to the edge of the page (Fig. 1) and the page gets vibrated off to the side, then one could easily end up with a narrow gap of white space down the edge (Fig 2).
The other side of printing to the edge is the trimming process. As the publication is being printed on larger paper, it must be trimmed down to the proper size. Again, this seems simple enough, but remember that the vibrations in the printing press have thrown things off a little so that each page is not necessarily centered in the exact same place as the other pages.
One could conceivably sit there and trim down each page individually going off the edge that has been printed on that particular page. This is not so bad when you have a half-dozen pages to trim. Yet it becomes a mite impractical when one is talking about trimming a few hundred or thousand sheets. If you base you cut off the top sheet of paper, it may or may not correspond with the sheets in the stack below.
The response to these printing issues involves what is called bleed. Bleed refers to carrying your background beyond the intended edge of the paper. Where I worked, we usually did this about a quarter inch (fig 3).
On the opposite side, we would make sure nothing vitally important (say the actual text, photos, etc.) was within a quarter inch of the intended edge of the paper (fig 4).
Together, this gave us some wiggle room so that if the vibrations threw things off, or the trimming was not lined up quite right it would be less likely to be noticeable. Setting this up however meant that one had to be aware of this process from the beginning of the design so as not to push anything important too near the edge.