Automatic Terminal Information Service, or ATIS, is a continuous broadcast of recorded noncontrol information in busier terminal (i.e. airport) areas. ATIS broadcasts contain essential information, such as weather information, which runways are active, available approaches, and any other information required by the pilots, such as important NOTAMs. Pilots usually listen to an available ATIS broadcast before contacting the local control unit, in order to reduce the controllers’ workload and relieve frequency congestion.

The recording is updated when there is a significant change in the information, like a change in the active runway. It is given a letter designation (e.g. bravo), from the ICAO spelling alphabet. The letter progresses down the alphabet with every update and starts at Alpha after a break in service of 12 hours or more. When contacting the local control unit, a pilot will indicate he/she has “information” and the ATIS identification letter to let the controller know that the pilot is up to date with all current information.

(This is an excerpt from Wikipedia, as I think this is the best explanation available.)

Here you can hear the ATIS from Stuttgart Airport (EDDS), Southern Germany that we recieve before landing and take-off: Information “Bravo”