Bleed is when a graphic image exceeds the paper's edges allotted to compensate for a margin of error that occurs during cropping and printing. For example, when printing business cards with a graphic touching the edge, if the image does not account for bleeding, there may be a white edge. The picture is an example of how a print with no bleed may look. As as you can see, the last example has a small white border on the left side of the image.
To create an image or another document that accounts for print bleed, it needs to be slightly larger than the printing area. The sizing is usually between 5mm and 25mm on each edge. The value varies on the printing press, how the image is sent to the printer, and the size of the paper.