It is a NOTAM (Notice to Airmen). If you are within 30 nautical miles of Denver International Airport and flying between the ground and 18,000 feet, you must have a flight plan filed with a specified transponder squawk code that can be monitored by the air traffic controllers. You must also maintain two-way radio contact at all time in that area. It appears that a Presidential visit on on tap for that time frame and extra security is being imposed.
Thanks!