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From navajocodetalkers.org:

DURING THE EARLY MONTHS OF WWII, Japanese intelligence experts broke every code the US forces devised. They were able to anticipate American actions at an alarming rate. With plenty of fluent English speakers at their disposal, they sabotaged messages and issued false commands to ambush Allied troops. To combat this, increasingly complex codes were initiated. At Guadalcanal, military leaders finally complained that sending and receiving these codes required hours of encryption and decryption—up to two and a half hours for a single message. They rightly argued the military needed a better way to communicate.
When Phillip Johnston, a civilian living in California learned of the crisis, he had the answer. As the son of a Protestant missionary, Johnston had grown up on the Navajo reservation and was one of less than 30 outsiders fluent in their difficult language. He realized that since it had no alphabet and was almost impossible to master without early exposure, the Navajo language had great potential as an indecipherable code. After an impressive demonstration to top commanders, he was given permission to begin a Navajo Code Talker test program.
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THE CODE THEY CREATED AT CAMP PENDLETON was as ingenious as it was effective. It originated as approximately 200 terms—growing to over 600 by war's end—and could communicate in 20 seconds what took coding machines of the time 30 minutes to do. It consisted of native terms that were associated with the respective military terms they resembled. For example, the Navajo word for turtle meant "tank," and a dive-bomber was a "chicken hawk." To supplement those terms, words could be spelled out using Navajo terms assigned to individual letters of the alphabet—the selection of the Navajo term being based on the first letter of the Navajo word's English meaning. For instance, "Wo-La-Chee" means "ant," and would represent the letter "A". In this way the Navajo Code Talkers could quickly and concisely communicate with each other in a manner even uninitiated Navajos could not understand.
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AFTER THE WAR, THE NAVAJO CODE TALKERS returned home as heroes without a heroes' welcome. Their code had been so successful, it was considered a military secret too important to divulge. They remained silent heroes until more than two decades later. Even after declassification of the code in 1968, it took many years before any official recognition was given. In 2001, nearly 60 years after they created their legendary code, the Navajo Code Talkers finally received well-deserved Congressional Medals of Honor.