What are Bleeds
When a job is printed it is allocated more space than is actually required for the finished item. For example an A4 leaflet would be printed in the middle of an area that is actually A4 plus about 4mm all round. This allows the artist to intentionally place objects on the page that run over the edge of the finished sheet. When the job is guillotined the surplus 4mm, known as trim, is discarded leaving a finished item with print right upto the edge. Page items that are placed so they extend over the guillotined edge of the final page are known as bleeds.
This technigue is used because it is impossible to guillotine in perfect position. It is important that any item that you want to bleed over the edge of your sheet extends 2mm beyond the edge as shown below.



Some programs do not allow you to place objects beyond the edge of the page. In this case you must make your page size bigger to accomodate the bleed area.