Very good explanation of how it works.
ATIS is short for Automatic Terminal Information Service, at major airports you can listen to a pre-recorded ATIS message at a special frequency where you can get important information about weather, winds, visibility, the runway/runways in use etc.
The purpose is to make ATC more efficient so the air traffic controller does not need to repeat these things for every single departing or arriving aircraft.
The message always start with, for example at LHR: "This is London Heathrow Information, Foxtrot,...".
As soon as something changes, for example the weather, the wind or the use of runways, a new message is recorded with the updated conditions, this message is given the next letter in the alphabet, in this case it would be Golf.
At first contact with the tower, ground or approach controller the pilot gives this letter to the controller, often you hear "American 123, Information Foxtrot..."
This means the pilot has listened to the ATIS message Foxtrot and there is no need for the controller to repeat any of the information contained in the ATIS message (and the controller knows the pilot has received that important information), if the message has changed the controller can tell the pilot that there is a new updated message available, you can hear for example "American 123, negative, information is Golf".
The pilot then knows the information he/she has is old and can switch to the ATIS frequency to listen to the new information.