Thanks to wikipedia-
Bleed is a printing term that refers to printing that goes beyond the edge of the sheet after trimming. 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. Bleed ensures that no unprinted edges occur in the final trimmed document.
Bleeds in the USA generally are 1/8 of an inch from where the cut is to be made. Bleeds in the UK and Europe generally are 2 to 5mm from where the cut is to be made. This can vary from print company to print company. Some printers ask for specific sizes; most of these companies place the specific demands on their website.