What is Bleed? Bleeds allow you to run artwork to the edge of a page. On a press, the artwork is printed on a large sheet of paper and then trimmed down to size. If you do not allow for a 1/8 of an inch bleed, any misalignment while cutting will result with the artwork not running to the edge of the paper. Bleeds ensure you get the results you need. Build your files 1/8" larger than the final trim size. For example, if you have designed a standard 3.5" x 2" business card with a red background covering the whole area, you will need to enlarge that red background to 3.75" x 2.25". This will make the red background extend 1/8" on every side of the page.
Why is adding a Bleed necessary? Small mechanical variations can end up leaving a hairline white edge where there should be no white edge at all, if the image is not extended beyond the final trim size. Extending images 1/8" beyond the final trim size guarantees that images truly will go all the way to the edge of the printed paper.