It's a virtual MIDI interface. Most software synths are designed to accept MIDI input from your soundcard from a keyboard or something. With a virtual midi cable, it's just that the "keyboard" sending the information is in your PC too; your midi app~
To your sequencer (most likely tabit for you), the virtual midi cable appears as another output device. You simply set tabit to play out through that, and your software synth to listen to input from it =)
MIDI Yoke NT is a free virtual midi cable, which you could use to connect tabit to synthfont.
I use the virtual midi cable that comes with the sw synth I use (VSampler 3).