A pencil moves across a sheet of paper, one letter at a time. The message is not a voice. It arrives as short beeps and long beeps, the dots and dashes of Morse code. The person at the desk writes each letter as it comes, so the whole line does not have to be held in the head. That work had a simple name. The message was being copied.
This is the usual story of a line still heard in movies. Someone on a radio says, "Copy that." It can sound like yes, or like I agree. In radio talk it means something narrower: I heard you, and I understood you. The words are short on purpose. In a noisy room, a short, clear reply is easier to catch than a long one.
Why copy, though, and not understand? In the early days, spoken radio was not how messages traveled. They were spelled out letter by letter in the telegraph code developed by the American inventor Samuel Morse in the early 1800s. The beeps were called dits and dahs. The receiver copied them onto paper, then sent a short reply to show the message had come through clearly.
Spoken radio largely took the place of Morse code in the 20th century. The pencil work faded. The word stayed. Copy, or copy that, became a quick way to say the message had been received. It is often used when more than one person is listening to the same broadcast. The job of writing every letter down is gone. The habit of saying so is not.