Airfield information on that ATIS (Automatic Terminal Information Service) broadcast on a radio frequency is letter coded to ensure that the current set of information is being used.
ATIS will go something like this "London Heathrow Information Golf: Landing runway 27L, wind 230 degrees 15 knots, visibility 5000 meters, few [cloud] 2000 feet, temperature +7, dewpoint +5, QNH (pressure) 1015..." When the details change, the letter is updated to the next one alphabetically. Passengers listening to channel 9 on United don't usually hear the ATIS, because the flight deck crew get it on another (secondary) radio.
In the above example, pilots may be asked to confirm they have "information golf" (G) - effectively they are being asked to confirm that they have the latest airfield/weather information. If the pilot says he has received foxtrot (F), he has received the previous report not the current one.
Andy
PS: Heavies - heavies are usually just the widebodies (can't remember the exact conditions for "heavy"), but 757 can be designated heavy as it produces more wake turbulence than many other narrow bodies