A bleed is when a graphic image or another element exceeds the edges of the paper allotted to compensate for a margin of error that can occur during cropping and printing. For example, when printing business cards with a graphic that touches an edge of the card, if the image does not account for bleeding, there may be a small, but noticeable white edge. The picture is an example of how a print with no bleed may look. As can be seen in this picture, the last example has a small white border on the left-side of the image.
To create an image or other document that accounts for the print bleed, the image needs to be slightly larger than the printing area, regularly between 5mm and 25mm on each edge. However, this value can vary depending on the printing press, how the image is sent to the printer, and the size of paper.