ATIS (Automatic Terminal Information Service) basics
Albert,
A pilot flying into an airport needs to know certain information like wind direction and what runway is in use as well as several other items. When you have just a few flights a day it is a simple thing for the tower controller to give the pilot(s) this information. However, if you have several hundred flights a day then it would become terribly cumbersom.
There is a dedicated frequency that gives the pilot(s) that information it is called the ATIS (Automatic Terminal Information Service). The information is updated hourly, and as necessary as significant changes take place.
When the pilot checks in with the approach controller he/she gives the flight number/aircraft number and the single letter designator of the ATIS information they have: A (alpha), B (bravo), C (charlie)-whatever. This communicates to the approach controller that this flight is aware of the current conditions and runway(s) in use.
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