The words Alpha Bravo Charlie refer to the NATO phonetic alphabet. It is also sometimes referred to as ICAO (International Civil Aviation Organization) radiotelephonic alphabet.
The NATO phonetic alphabet is used to facilitate radio communication by providing a standard phonetic transcription and set of code words. It allows for communication across language barriers and can even be used in everyday circumstances when an important word or name needs to be spelled out over the phone or radio. By using the code words to indicate letters, confusion caused by phonetically similar letters (such as "d" and "t" or "b" and "v") can be easily avoided.