A "Bleed" is a printing term that describes the fact that you need any solid photos or photographs that go all the way to the end of the page need to be extended out past where the paper will be cut. The reason for this is that there is slight movement of any sheet of paper in the printing and cutting process. If you don't create art with bleeds, then you are likely to have a white line at one or more edge of the sheet where you intended it to be a color or photograph.
In our example below, the crop marks or cut lines have been indicated in red. (They will always be black in a production environment.) Note how the color extends beyond the crop mark. Common bleed are .125 beyond each edge. If you were designing an 8.5 x 11 tri-fold full color brochure, you would have a design sized 8.75 x 11.25