Bleed is a printing term that refers to printing that goes beyond the trim edge of the sheet.
If you want your image to print right to edge of the page or 'edge-to-edge' drag the image frame past the dotted trim line into the bleed area.
When an image is only dragged to the dotted trim line an unwanted fine white boarder may be visible on the finished page.
If you wish to have a white border at the edge of the page, drag the image frame a few millimetres from the trim edge. When the page is trimmed a thick solid white border will be visible.