HF/DF or 'High Frequency Direction Finder' (Better known as 'Huff-Duff') was a system implemented by the allies in 1942, that could track down the position of U-boat radio stations.
The technology itself was already used for low frequency stations before WWII, but it was the Royal Navy that designed the first high frequency system. Radio signals are received by a HF/DF (usually shore-based) installation, which would track the direction the message was sent from. The bearings from several HF/DF installation could then be triangulated, providing a reasonable fix on the sender's position. Nowadays this technology is mainly used to track down illegal radio stations.
Because the German U-boats sent out a homing signal to the pack leader regularly to keep the pack together, the allies were able to roughly track German wolfpacks, which gave the convoys a pretty good idea which area to avoid. The allies recognized the power of this system, and employed it all over the globe.
When a U-boat pack was located by a HF/DF station and close enough to be in sonar range, nearby hunter-killer groups (and sometimes convoy escorts, when the pack was especially close) then could then locate the exact location of the pack by sonar and attack it. About a dozen U-boats were sunk just after sending a message home. A U-boat radio silence protocol for wolfpacks was never issued though.
Many thanks to The Custodian for his comments and suggestions.