In printing terminology, the bleed is the extra margin on a print product that is meant to be trimmed off when the product is trimmed to its final size. Bleed basically means that the color and graphics on a product "bleed off the page" to ensure continuity of the color to the edge of your product.
A full bleed is required for all print-ready files. For example, if you are ordering a metal print with a size of 20" x 30" your final file should be 20.3" x 30.3" a bleed of 0.3" is added in height and width.