Bleeds refer to printing that goes to the edge of the sheet after trimming. If you intend for an image to print all the way to the edge of the paper, it should extend 2mm beyond the crop line. The extra 2mm that will be trimmed away is known as the "bleed."

Bleeds are necessary because although modern trimming machines are capable of great precision, they are not perfect.  If you create an image that is precisely the same size as the finished piece, and the press sheet shifts 0.2mm while being cut, one side of the image will have 0.2mm cut away, and the opposite side will have a white "flashline, 0.2mm wide.  By creating auxiliary image area that is intended to be cut away and discarded, you can avoid flashlines.

When creating your artwork size, add an additional 2mm to each side. For example, Business cards 86mm X 54mm, increase size to 90mm x 58mm. (2 mm on each side)