I've described it before - but here goes again!

FRED - Flashing Rear End Device

Used on American railroads - mounted on the last (rear) car of a mainline train. Used to monitor air pressure in the brake line and will automatically slam on the brakes for the entire train if the air pressure is lost. When FRED senses a problem in the train ahead of it - it causes the entire train to come to a halt.

So - in the bicycling world a FRED is the guy in the Peloton who can, by some weird action, cause the entire Peloton to come to a screeching halt

Hope this helps