I just know chatter is something related to talking.
Counter-terrorism police in Sydney have arrested two men, charging one with possession of documents designed to facilitate a terrorist attack. The arrests were related to a series of counter-terrorism raids in September, officials said. They come after Australia's prime minister said there had been heightened "terror chatter" since a cafe siege in Sydney last week. (BBC)
In this context it means "an increased quantity of messages sent by suspected terrorists and terrorist sympathisers to each other", as detected by intelligence services.
A quote from Herald Sun:
AUSTRALIA’S spy chiefs have detected an increased level of “chatter’’ among terrorists, sparking a grim pre-Christmas security warning from Prime Minister Tony Abbott.
The security services monitor the Internet, the cellphone networks and other means of communication. They have lists of people suspected of being terrorists or of harboring sympathies to terrorists. And they have discovered that in the recent days these people have been sending more messages to each other than previously.
I wasn't sure what the phrase meant but a short googling for "terror chatter" (with quotation marks) helped me to find that out rather quickly.
Chatter means conversation, as above, or in a different context, vibration of teeth, as when cold, and by extension, unwanted mechanical vibration.
