It's called 'High-Frequency Direction Finding' (Huff-Duff) and was used from shore stations to begin with, then later, when the sets were small enough, from surface ships. By 1943 it was common for at least two convoy escorts, plus the rescue ship at the rear, to carry this device.
It didn't read or decipher the messages, it just pin-pointed the source of transmission. By using simple triangulation it could cross-reference the point of a 'talkative' U-Boat and then friendly forces would be homed onto it. The Germans never understood this and abused the radio right up to the end of the war.
Google 'huff-duff' and read the wiki.
Remember the old war sayings: 'Careless talk costs lives' (UK) and 'Loose lips sink ships' (USA). In this case, they are your lives.
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