The bleed margin (lost border) refers to the 5 mm gray border around your document in the online editing application.
The pages are cut with a precision of 3 or 4 millimeters. This means that only part of this gray area will be cut.
You can cover this area entirely if you do not want to have a white border around your layout, part of the photo will then be cut (3-4mm).
Do not do this if you have a face close to the edge or text.