It is a MacOSX resource fork.. its essentially a file that holds "info" about other files.. for example, it might hold the icon for a folder, or which application should open a file.. or where this file's icon should appear on the mac desktop

you can delete it, but your mac will re-create it next time you use usb mass storage mode..

failing that, just ignore it, it won't cause you any problems..

Macs don't work like other computers :P