In printing, a “bleed” is where your background color or image extends all the way to the edge of the sheet. This must be printed on a larger sheet and then trimmed down. In other words, the bleed is the area to be trimmed off. After trimming, the bleed ensures that no unprinted edges are in the final trimmed document.
In practical terms, if you are producing the artwork yourself use crop marks that define a print area. Bleed is the term used for the extended area of your artwork that goes beyond the crop marks. Usually 1/8″.