When you set up a new document in Adobe Illustrator one of the items you have to specify is the bleed guide.This is only really relevant in the context of print media, and is one of the many topics we cover in our Illustrator courses. 
Bleed is a term used in the printing industry for the safe area outside the document, and it ensures that the artwork will be printed to the edge of the paper, card or vinyl. Most printing companies specify between 2 and 5mm bleed and you should always check with your printing company first before sending finished artwork to print.
The reason bleed is required is that many documents like flyers and posters are printed 8 or 16 to a sheet, and these are trimmed with double-edged high-speed trimmers. However, the trimmers are not 100% accurate, and if the artwork was taken only to the edge of the document (the black trim edge), you might end up with a small thin white strip. By scaling your artwork up to the red bleed guide, you will ensure that it’s printed to the paper edge.
The bleed guide is usually specified in the dialog box when you create a new document in Illustrator. If you forget, or need to adjust this later, you can do so in File > Document Setup. When you create the PDF in order to send to print, the dialog box has a section named Marks & Bleeds. Tick All Printer’s Marks and Use Document Bleed Settings.
By attending to details like this, you’ll ensure that you have complete control over the results and your artwork is printed to a professional standard.
Whether you’re a complete beginner or self-taught, our courses will help you gain self-confidence in your Illustrator workflow, as well as adding to your professional skill-set. See many examples of our Illustrator clients’ work on our Facebook page.
Other related Illustrator Blog Posts:
Illustrator workspace
Illustrator basics
Illustrator clipping masks
Illustrator live trace
Illustrator brushes