It is simply a means of calling your own AV to scan the USB when you plug it in.
You have to have the usbvirusscan program running (either on boot or manually started) so that when you plug in a usb drive avast would scan all of it automatically and not just files that are run. The file used to do the scan is C:\Program Files\Alwil Software\Avast4\ashQuick.exe, that is what has to be entered in the usbvirusscan settings.
Personally I don't like having the usbvirusscan program running all the time, when it is a very rare that I plug-in a USB and more so one that has been connected to another computer; so I start usbvirusscan just before I would plug in a usb of a suspect source (e.g. one that I haven't had total control over).
There are downsides too as if you had the usbvirusscan running and you plugged in an external USB Hard Disk then ashQuick.exe would scan that, could be waiting a while for that to complete