In printing, bleed is the area where the page will be trimmed. If you intend your image to be at the edge of the page, the image must extend beyond the bleed line into the bleed area. The actual bleed line may not be in the same position every time so it is important to extend the image well beyond the bleed line. Otherwise, your page will result with a white border.
The photo is extended beyond the red dotted line.
The photo is not extended beyond the red dotted line. When printed, this page will have a white border.