Bleeds allow you to run the design over the edge of a page. On a press, the artwork is printed on a large sheet of paper and then trimmed down to size. If you do not allow for 3-5mm of bleed, any misalignment whilst cutting will result in the artwork not running to the edge of the paper. The edges of your trimmed final product will most likely end up white.

Below is an image that shows the white (Your artwork size) with another outer border that is filled with design elements. These elements will be cut off during production.

Be sure to avoid adding text to the edges of the artwork as this will be cut off. Leave an inner margin, on your artwork, of 3-5mm clear of text but be sure to extend the design.
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Bleed = Product size on specification - 6mm (3mm for each size)

Inner Margin = Product size on specification - 6mm (3mm for each size)