When the network news talks about the category of threat they sometimes mention “increased chatter”. What are they talking about? Do they monitor phone conversations, email, or what? Are they listening to only suspects or do they monitor the volume of various kinds of communications? Is there a chatter index or something?
Of course they <three letter orgs>FBI CIA NSA</three letter orgs> monitor conversations - email, radio, telephone. Many of these are highly (> 128 bit DES) encrypted - which if the NSA can de-encrypt - they can’t do it in real time.
They also monitor internet activity. Some folk - maybe even you - were checking out US sports stadium layouts in the past week. Apparently Al-Qaeda types were too - thus the Very Vague Warning the FBI issued.
Chatter is - as you suggested jacksen9 a measure of volume of traffic from/to BadGuys. While it may not take much effort or bandwidth to transmit the cryptic ‘The son of Saddam flies in the Sunshine State’, other plots involve lots of big graphic files and replies. The FBI has noted an increase in activity in the days leading up to every terror plot. This is the government, so you can bet there’s a “chatter index” and a classified milspec for its definition.
Now that the FBI makes it known they are monitoring “chatter” and releasing VeryVagueWarnings based on it, it’s natural to assume the BadGuys leave their computers in the SPAM setting - sending ‘Worst Episode Ever’ missives to eachother, with Captain Janeway JPEG attachments. 
The official name of what Corbomite is explaining is “traffic analysis”. Just by watching the from/to/how many messages of the people you are watching tells you a lot. Even if you can’t decrypt it. People with good crypto keep up a steady stream all the time, using dummy messages to fill any voids that occur. This takes a lot of money and sophistication.
The Bin-Laden gang isn’t that good. (They in fact used little or no crypto prior to 9/11.)