Simply put, bleed is an additional 3mm at each edge of your artwork. So, an A5 flyer measures at 148mm x 210mm but, with bleed, it becomes 154mm x 216mm.

Why is this necessary? Well, this 3mm overprint acts as a safety net 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. 

Safe Zone, is the space being allowed in the other direction, between the edge and your text.