Automated Terminal Information Service, more commonly known as ATIS, is a continuous broadcast of recorded information pertaining to a particular airport area. ATIS feeds pilots essential information about their destination, such as weather information, which runways are in use, and any other important notices required by inbound pilots.
The recording is updated throughout the day as significant information changes, like information about cloud cover in the area or a change in the active runway. The new information cycle is given a letter designation (e.g. delta) from the phonetic alphabet. Every time the information is updated the identifying letter progresses through the alphabet, starting from Alpha at the beginning of each day.
Before a contacting the local tower at a destination, the pilot will listen to the ATIS broadcast and report that they are familiar with the current local information to the tower by stating the current letter designation.