Plain English Meaning
An area on the ground where things like live firing or other risky operations happen, but the people running them must stop the moment any other aircraft comes near. Because the activity is paused for traffic, other pilots don't need to avoid the area, and it isn't shown on charts.
Full Definition
A type of special use airspace containing activities that, if not conducted in a controlled environment, could be hazardous to nonparticipating aircraft. The distinguishing feature of a CFA is that activities must be suspended immediately when a spotter aircraft, radar, or ground lookout detects an approaching aircraft. Because nonparticipating aircraft are not required to change course or alter their flight path, CFAs are not depicted on aeronautical charts.