What do margin and overlap mean?

Most printers cannot print to the entire page area and hence margins are needed. If you know the margins for your printer, enter their width. The default, 10mm, is a safe bet and should work with practically all printers.

Overlap (or bleed) makes it easier to cut away the margins. The image overlaps at the edges of adjoining sheets. This means the cut does not need to be very precise, it will be sufficient if the path of the cut just goes along the overlap area (though straight cuts may look better). Note that the overlap is placed in the margin area, in other words, adding overlap does not effect the poster size.