Its a metaparameter,
I have never been inrigued by this before but after a quick bounce around google it turns out that when running something like xmms, it would be nice to have a file. so %F invokes it with n arguments (file name), Then when its xmms is running and you double click on a song on the playlist xmms %F changes to xmms filename.mp3 or whatever. well thats my understanding from what i just read, the link is here
http://cantonlinux.org/pipermail/tec...ch/002781.html
Would check it but work despises linux