Bleed is a commonly used technique in print design.
A commercial printing press can’t print to the edge of a sheet of paper. Instead, multiple products are printed onto a larger sheet of paper and cut down to size.
If a background image or colour extends right to the page edge, a designer will extend the element beyond the page boundary. The extension (usually 3mm) beyond the border is called ‘bleed.’
It is impossible for a guillotine to cut right down the edge of a design perfectly every time, due to a small margin of feed error. If a page did not have bleed and the guillotine blade cut a fraction outside the page border, a hairline white line would be visible instead of the image carrying all the way to the page edge. Bleeding background elements like colours and pictures beyond the trim border corrects this problem. Crop marks printed outside the artwork area help the cutter by displaying where the page ends and the bleed begins.