Before World War 2, the Germans had been using the Enigma Machine for a while. This caused many problems for the British and French as we could not understand the German signals.
The Enigma machine would subsequently be used by the German Panzers as they stormed across Europe - the device being relatively small and easy to use. Varients of the device would be used by the German Navy, and an understanding of the Enigma would prove critical in the battle of the Atlantic. Without this understanding the UK may well have had to surrender due to lack of supplies, we were that close to the edge.
Of course, all of this was in the future. All we knew is that we had to crack the enigma. Unfortunately we had never seen one. It was not possible for the British or French to work out the wiring of the machine, and hence the code, just from the messages. One of the main problems was that they didn't know the connections from the keyboard to the first rotor... did the A key go to the first, second, third terminal? What about B?
Fortunately the Polish cryptographers had reconstructed a machine, and in a secret meeting shortly before Poland was invaded they took the British and French into their secret cryptography facility and showed them their reconstruction.
Alfred 'Dilwyn' Knox, who had been puzzling about the 'keyboard wiring problem', was almost disgusted to learn that the wiring solution was the simplest, A went to the first terminal, B to the second and so on. This dramatically reduced the number of possibilities, and if this had been different the Enigma may not have been cracked.
The Poles also showed a prototype of the Bomba, a machine which allowed keys to be found more rapidly than by simple trial and error. The Bomba (named after the polish word for ice cream) allowed an Enigma key to be found in a couple of hours. Once found messages could then be decrypted.
Given that the both the allies and the Germans were working on the atom bomb, called the machines 'Bomby' could have been fatal. Had that one word leaked tattached to a location then the site in Poland, or Bletchley Park in the UK would have been bombed into oblivion.
The Germans had introduced new rotors, and the Poles were no longer 'current', but the information they provided gave the step up required for the British (and French) to continue. Eventually Bletchley Park was routinely cracking the German Enigma messages. This enabled our convoys to often avoid the German submarine wolfpacks in the Atlantic.