What is bleed?
Bleed is the portion of the print piece that goes past the edges of the paper and will be trimmed off when it is cut to the final size.
Why bleed?
Bleed ensures the continuation of color, images and design past the final trim size to prevent imperfections from possible cutting shifts. We require ALL pieces to have at least .125” (1/8”) of bleed on all sides that trim. We will automatically trim the bleed off the edge, resulting in your desired trim size.
How to add bleed to a document
We suggest you use InDesign for your print layout files. To add bleed in InDesign, you can go to File > Document Set Up > Bleed. In the box, type .125. Make sure it is copied to each side that will be trimmed. After your document has bleed, extend any image or color that goes up to the edge into that bleed area. Note: Make sure to keep text about .25" away from the edge of the trim to ensure it will print properly.

Add bleed when exporting a PDF
When you export the file as a PDF, make sure to go to Marks and Bleed and check Use Document Bleed Settings and Crop Marks. The crop marks will show you and the printer where the document should be trimmed. You do not need to check "Bleed Marks," but you can if you wish.

Troubleshooting:
If you're in need of more detailed assistance, you can check out Adobe's direct help center on bleed on their website.