Bleed explained

Bleed is the area an image extends to beyond the canvas size. e.g a design for an A4(210mm x 297mm) poster with a 3mm bleed will have the dimensions 216mm x 303mm. Bleed is necessary so that when your job is printed and cut to size there will be no white border.

When preparing your image you should design with bleed in mind. Start off with a canvas 6mm wider and longer than the actual printed size. Then make sure that background elements or images extend beyond the printed size into the bleed area.Make sure that any part of the image you do not want trimmed does not extend into the bleed area.

Below is an example of an image with bleed.

The lines in the corners are called trim marks and indicate where the image will be cut.You'll notice that the parts of the image removed are unimportant. They are mostly the red background and the white space of the image.The finished poster will look essentially as intended.

The bleed allows the image once trimmed to extend to the entire width and length of the page.