Bleeds are an important part of any design intended for print. These are the parts of a design that stretch all the way to the edge of the printed page. A printer will need these elements that bleed to go past the actual sheet size so they can then trim through that element creating a clean edge through the imagery. Without this being setup properly, the finish size of the document may end smaller than expected in order to achieve the desired bleed effect. There may also be some undesirable white edges where the image just didn't quite go far enough to get caught in the trim or content that you didn't want cut can inadvertently get into the trim because there simply wasn't enough space in the art. Let's say your building a new business card with elements that bleed. The standard finish size of a business card is 3.5" x 2". The PDF that you submit to your printer will be 3.75" x 2.25". This extra space gives the the printer an 1/8" on all four edges for trimming. Perfect!
How you get to these dimensions depends on the application you're using to do your design. Professional apps such as Illustrator allow you to preset your bleed space separate from your document size. This makes it very easy and convenient to add bleeds to any document. Simply select "Use Document Bleeds" when saving to a PDF and they will be included in your newly created file.
If you're not using an app that allows for this, you'll need to set your document size to include the extra 1/8" on all four sides. We then recommend you use guides that will show you where the edge of the page is after cutting so you can place your content appropriately.
In a nut shell, everything that bleeds goes past the edge of the page by 1/8" and everything that doesn't, such as typographical content, stays 1/8" inside the edge of the page.