Bleed is a printing term that is used to describe a document which has images or elements that touch the edge of the page, extending beyond the trim edge and leaving no white margin. When a document has bleed, it must be printed on a larger sheet of paper and then trimmed down.
The example below shows how a document should be set up with bleed. This is the way you must set up your document if you intend to have graphics that extend all the way to the edges of the finished item.
The example uses a business card, however the same principle applies to a document of any size.
No printers can print right to the edge of a sheet therefore any standard size prints with bleed must actually be printed on a larger sheet and cut down. For example, an A4 page with bleed must be printed on an SRA4 sheet (or multiple pages on an SRA3, or SRA2 etc) and then cut to size.
This short film explains how to create bleed in Adobe Indesign.
See also Paper Sizes