This option allows you to select a MIDI device. The list of devices varies, depending on what MIDI hardware is installed. If your PC doesn't have a MIDI device, the only option is "No MIDI input", which disables FFRend's MIDI support.
The default value is "No MIDI input", which means that in order to use MIDI, you must select a MIDI device at least once. You'll also need to assign specific MIDI messages to FFRend's functions, using the MIDI Setup dialog.
If you're using a USB MIDI interface, it's better to connect it to the PC before launching FFRend. If you launch FFRend while the interface is disconnected, your device selection will be lost. If you connect the interface while FFRend is running, it won't automatically appear in the device list; you must press the Refresh button to refresh the device list. If that doesn't work, exit FFRend, reconnect the interface, re-launch FFRend, and select the device.