What is Bleed?

Bleed is the printing term for the additional image area added to a document for trimming. It is usually recommended that a professional document has 3mm of additional image area past where it trims (the artboard).

Despite print guillotines being relatively accurate, there is always the chance of a stack of paper shifting a fraction when the clamps applies pressure prior to the blade cutting. If a document didn't have any additional image area (bleed), than the final document would have a white hair line down the side.