Bleed is an essential print area outside the document that must be added to your artwork before it's sent off for print. It is an extension of an image or color(s) that continues beyond the finished trim size.
The industry standard of bleed is to add an extra 0.125 inches or 3mm extension to the artwork. The purpose of a bleed area is to guarantee that, in case of any slight shifting during the printing and finishing process, the paper's natural color will not show on the edge of the finished print when trimmed.
Below images show how bleed is applied on a template and an actual print-ready file:
(Bleed appiled in a template)
(Bleed appiled in an actual print-ready file)