SIGINT stands for SIGnals INTelligence, which is intelligence-gathering by interception of signals, whether by radio interception or other means.
SIGINT is actually a broad discipline under which multiple sub-disciplines fall. There are three major sub-disciplines that fall under SIGINT which are COMmunications INTelligence (COMINT), ELectronic INTelligence (ELINT), and Foreign Instrumentation Signals INTelligence (FISINT). It should be noted that COMINT is commonly referred to as SIGINT, which can cause confusion when talking about the broader intelligence disciplines.
SIGINT became far more central to military (and to some extent diplomatic) intelligence generally with the mechanization of armies, development of blitzkrieg tactics, use of submarine and commerce raiders warfare, and the development of practicable radio communications. For example, failure to properly protect its communications fatally compromised the Russian Army in its advance early in WWI and led to their disastrous defeat by the Germans under Ludendorff and Hindenburg at the Battle of Tannenberg. Similarly, the interception and decryption of the Zimmerman telegram was an important factor in the US decision to enter the War.
On the negative side, the British commanders' lack of ability to take seriously traffic analysis information from intercepts was instrumental in the failure to achieve more than they did at the Battle of Jutland, thus losing what might have been a major opportunity.
The use of SIGINT had important implications during WWII as well. Early on, Admiralty dismissal of SIGINT information (also traffic analysis in this instance) contributed to the loss of HMS Glorious in 1940. The Allied ability to intercept and decrypt the German Enigma "E Traffic" and Japanese Purple (Magic) traffic proved to be a great military advantage. The combined effort of intercepts and cryptanalysis for the whole of the British forces in WWII came under the code name (Ultra) managed from Government Code and Cipher School (Bletchley Park). Perhaps most dramatically, intercepts of Japanese naval communications yielded information that gave Admiral Nimitz the upper hand in the ambush that resulted in the Japanese Navy's defeat at the Battle of Midway, six months after the Pearl Harbor disaster.
As sensitive information is often encrypted, SIGINT often involves the use of cryptanalysis. However, traffic analysis--the study of who is signalling whom and in what quantity--can often produce valuable information, even when the messages themselves cannot be decrypted.
See also: ECHELON.