The Phonetic Alphabet is an easy substitution cipher used by airplane pilots. It is a way of representing a letter with an entire word. This is used often when communicating letters can be ambiguous, e.g. reading the serial number of your new cellphone to an insurance company over a bad phone line. You will know some words already, such as Alpha, Bravo, Charlie.
The main reason for using it is that some letters sound really similar (like B, P, D and T), and can easily get confused over a bad connection, but the words that represent these letters sound quite different (Bravo, Papa, Delta, Tango) and can't be confused so easily.
Here is the alphabet: