Bleed: Is the term used for objects that overlap the border of your document.
See the orange rectangle in the below image. If you imagine the grey rectangle to be a typical A4 page size on the pdf; an image that bleeds correctly continues off of the page past the grey rectangle. For backgrounds that you want printed flush to the edge, make sure that your PDF background goes to the edge of the page.
When documents are uploaded without bleed we will upscale your artwork to create bleed typically at 1%. This then allows our printers to cut to the edge of the page avoiding the risk of any unwanted white borders. We advise that all PDFs should be supplied with a 3mm bleed around. Bleed ensures that your artwork images will be cut to the edge without a risk of a white border or cutting off wording/images.
Crop Marks: If you are able to, always include crop marks on your PDF as this tells the printer where to trim your artwork with correct trim sizes. Crop marks look like right angles in the corners of your PDF as shown below:
Page layout: Always leave a 3mm gap from the edge of the artwork to avoid text and images from being cut off.