I think he means something that monitors the feed and figures out what % of time is being used by radio traffic and what % is dead air.
So a really busy feed, with lots of radio traffic, might be like 75% actual transmissions and 25% dead air. A really slow one might be 1% traffic and 99% dead air.
And then putting that number, updated however often, right in the listing of feeds.
That way someone could look and see what streams actually had the most traffic on them quickly.
Sounds kinda hard to do though :)
Or if I have it totally wrong, maybe something like this FAA AIRPORT ARRIVAL DEMAND CHART would be closer to what he speaks of:http://www.fly.faa.gov/Products/AADC/aadc.html