What is Bleed?
Bleed is the term used for ink that extends past the edge, or trim size - of the page. On any press, there will always be a small amount of movement when a job is printed. If your artwork is only the exact size of the page, you may see a fine white line at the very edge when the page is trimmed. To prevent this, you should always add 3mm bleed to your artwork; that is, make your artwork 3mm wider all round than the trim size of the page.
How do I apply Bleed?
When creating your artwork in your chosen desktop publishing programme, any graphics or images that go to the very edge of the page (known as bleeding off) should be extended at least 3mm past where the page will be cut or trimmed. Then, when creating your print-ready pdf, make sure you set your bleed margin to 3mm.
Some programmes like Adobe Photoshop, Microsoft Word and PowerPoint don't have the facility to add bleed when creating your pdf. This means that when creating your artwork, you have to make your page or image size 6mm larger than the finished job. The 6mm (3mm all round) will then be treated as the bleed when the job is printed and trimmed off when the page is cut to size.