On some printers it is difficult to print exactly to the edge of a sheet so, to achieve this, it is necessary to print a slightly larger area than is needed and then trim the paper/card down to the required finished size.
Bleed refers to printing that goes beyond the edge of the sheet before trimming. The bleed is the area to be trimmed off, which gives the printer a small amount of space to accommodate for movement of the paper, and printing inconsistencies, and to ensure that no unprinted edges occur on the final trimmed document.
Bleeds are generally 1/8 of an inch (3mm) from where the cut is to be made, but this can vary from one print company to another. Some printers ask for specific bleed sizes, and if so will typically place the requirements on their website.
When designing a document, images and fills which are intended to extend to the edge of the page should be extended across the full width of the bleed.
Crop marks are added to show the finished document size, and indicate the guillotine cut positions for trimming off the bleed.