Here's a pretty good definition of QoS:
The short answer is - QoS stands for "Quality of Service".
What that means practically, is that a router that supports good
QoS capability allows you to set it to give a higher or lower
priority to certain types of traffic vs other types. In the context
of Vonage and VoIP, a QoS capable router like the WRT54G or WRT54GS
allows you to set the VoIP traffic from your Vonage adapter to have
a higher priority than traffic from websurfing, downloading or
uploading files, or other activities. So, if you're doing many
things on your internet connection at the same time, it will make
sure that your Vonage calls still happen without choppiness or
cutouts, even if it means slowing down your file transfers or the
speed at which you are surfing and loading webpages.
- DallasFlier of Vonage
Forums
And I don't think it is as difficult as you imagine. As how one of
the users have described, it seems like you're just a cut-and-paste
job away from getting it working.
You simply run the tool, cut the output and paste it into your
routers firewall rules screen.