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.
Uhhhh what? I don't know if I've ever felt chatter while flatbasing.
It happens to me on my heelside when I'm trying to do a good solid carve on relatively hardpacked groomers. Instead of biting and carving, the board will slightly lose grip and "chatter" sideways over the surface. It's not a smooth skidded turn, it's a failed attempt at a proper carve with no skidding.
For me the fix was to bend my knees more on my heelside. Don't know why it worked, but it did!