An image bleed in printing refers to the extra printing that goes beyond the area that will be trimmed. The bleed is the area that will be trimmed off. A bleed is used to ensure that no unprinted edges occur once the document is trimmed to it's final size. Cutting a document to exact dimensions can be difficult, and so bleeding allows for this movement and inconsistencies in cutting.

As an example, let's say your final poster size will be 13" x 19". Your poster has an orange background. If you print without a bleed you may end up with thin unprinted (white) lines on the edges of your paper if the cutter has difficulty cutting exactly 13" x 19" square. If, however your document is designed with a .0625" bleed on each side your final document will be 13.125" x 19.125". Extending your orange background to the bleed dimensions will ensure that when the printer cuts your document down to 13" x 19" the orange background will fill the whole document and you will not have any unprinted areas where the cut happens.

Bleeds can vary in size from print company to print company, but generally are between 1/8" and 1/4" in size.

Similarly, the "Safe Area"of a design ensures important text and images won't get cut. The safe area extends inside the final document size instead of outside like a bleed does. Because of the same inconsistencies with cutting, a safe area is wise to eliminate the chance of cutting your images or text if you design too close to the edge.