When an image needs to be printed all the way to the edge of the page or package without any white excess border, it needs to have an extra printing on a larger piece of paper that will be cut off because printing to the edge without cropping is extremely difficult. This extra space that gets cropped off is the bleed. In the US the bleed is typically 1/8 of an inch, while in Europe is it 2-5mm. These measurements may change based on printer specifications.