Many home printers cannot print all the way to the edge of a piece of paper.
If Octavo knows which printer you are using (selected in the Media tab), it will display a red shaded area on the Placement views, showing where your printer cannot print. Make sure that crucial content does not fall into this area!
If you have a printer with this limitation but you want to print edge-to-edge, you might want to trim your printed output, i.e. print on a larger sheet of paper then cut it down to size. That way, your content won’t need to worry about the printer’s unsafe area.