The International Radio-telephony Spelling Alphabet, also known as the phonetic alphabet is the most common spelling alphabet used for voice messaging.
It was developed within aviation. Each letter of the alphabet was assigned a code word that is distinct from the others. These words were designed so that they could be understood over radio or telephone. This is regardless of language differences or the quality of the channel.
The RYA radio course requires that you can spell any word phonetically using the correct words from the alphabet.