FFRend supports a dual-monitor setup, in which its main window and output are displayed on two different monitors. In the usual case, the main window is on the primary monitor (the built-in display on laptops), and the output is on the secondary monitor, but the reverse also works.
To make FFRend use a secondary monitor, you must first extend your Windows desktop onto the secondary monitor. This is typically done using Control Panel/Display/Settings, or in some cases via software that came with your graphics card. Once you've extended the desktop, you can drag FFRend's output window onto the secondary monitor. Now when you enable full screen mode, the output window fills the secondary monitor.