Chatter is a signals intelligence term, referring to the volume (quantity) of intercepted communications.
Intelligence officials, not having better metrics, monitor the volume of communication, to or from suspected parties such as terrorists or spies, to determine whether there is cause for alarm.
[2] Intelligence specialists hope to learn significant information by methodically monitoring when and with whom suspects communicate.
These considerations do not apply when the targets of analysis follow the military practice of maintaining a steady flow of encrypted communications whether they are needed or not.
[3] Some events, including the capture of the "Algerian Six", were triggered largely by an increase in "chatter".