What is Bleed?
Bleed is printing that goes beyond the edge of where the sheet will be trimmed. In other words, the bleed is the area to be trimmed off. Artwork and background colors often extend into the bleed area. After trimming, the bleed ensures that no unprinted edges occur in the final trimmed document.
It is hard to print exactly to the end of a sheet of paper, it is needs to print a little bit extra area than is needed and then trim the paper/card down to the required finished size. Images, background images and fills which are intended to extend to the edge of the page must be extended beyond the trim line to give a bleed.
Simply put, bleed is an additional 3mm at each edge of your artwork. This 3mm overprint acts as a safety in case the guillotine blade doesn’t come down exactly on the edge of your artwork. Without bleed, the guillotine might cut ever so slightly to the left and you’d have a slither of white appear on the edge of your product.