Bleed is required when graphics continue to the edge of a sheet of paper. A commercial printing press can't print to the edge of a sheet. Multiple products are printed on much larger sheets of paper and then cut down to size. A little 'over print' is required, and this is known as 'bleed'.
'Bleed' can be described as an area of tolerance. The industry standard requires 3mm of 'bleed' on each edge and a 3mm safe zone. The safe zone is the 3mm inside the cutting edge in which no text or important information should be placed – they risk being clipped. This means that the length of each side will be 6mm longer. An A4 sheet with 'bleed' will be 216mm x 303mm. Its finished size will be 210mm x 297mm.