Chatter is the nose and/or tail of a board rhythmically slapping (that's what she said...) the snow as you ride, generally happens at speed while riding flat based.
The phenomenon is usually a resonance in the board due to the combination of stiffness and dampening, mostly a soft board with little or no dampening.
Take a spork, hold the bid end, and flip the small end with your other hand. It will vibrate. Same thing.
Drive a car with worn out shocks and hit a speed bump, same thing.