When any image or element on a page touches the edge of the page, extending beyond the edge,
If any element on your document layout makes contact with the document border you will have to use bleed. The trick is to place the element so that it goes over border where the document will be cropped after printing.
The term bleed is used for all objects overlapping the border of your document. Let’s say your working on a brochure with images against the sides of your pages. You’ll supply the printer with a document somewhat larger then the final document will be.
After the brochure is printed it will be cropped to it’s correct size. The bleed in your document gives the cropping some room for error. The paper itself can expand or contract, the cropping machine could setup wrong or the person working on the brochure could make a mistake. There are a lot of factors that could go wrong with the cropping, if you wouldn’t be using bleed the images wouldn’t be neatly aligned with the side of your printed document.
Two kinds of bleed
A bleed can be a full bleed or partial bleed. With a full bleed you have objects running of your document on all sides. With a partial bleed you’ll have a couple of elements running if the document.