Bleed is basically the part that the printer trims away when a print is finished. This is needed for all most of the commercial printing in order to achieve a seamless, borderless look.

Almost all the artworks and posters we create require a bleed. Here is an example:

For this reason, always leave enough room in your design for trimming. If in doubt, just speak with your graphic designer or our prepress team regarding the bleed requirement.