The bleed is the part on the side of a document that gives the printer a small amount of space to account for movement of the paper, and design inconsistencies. Artwork and background colors often extend into the bleed area. After trimming, the bleed ensures that no unprinted edges occur in the final trimmed document.
It is very difficult to print exactly to the edge of a sheet of paper/card, so to achieve this, it is necessary to print a slightly larger area than is needed and then trim the paper/card down to the required finished size. Images, background images and fills which are intended to extend to the edge of the page must be extended beyond the trim line to give a bleed.