Including bleed in your print artwork ensures that the finished product will not have any white boarders when they are cut. 


Basically, bleed allows you to run your artwork to the edge of the page. 


If you have any images, graphics, background colours that extend right to the edge of the file, you need to make sure the go an additional 3mm past the trim line. 


In a printing press, the artwork is printed on a large sheet of paper and then trimmed to the required size.