score:3
It's probably referring to the same thing as exists nowadays and your guess is correct.
When transmitting a signal through a medium (wire, air, or fiber optics nowadays) over long distances, you need repeaters every so often to boost its amplitude and overcome attenuation. In some specific cases you could also change the medium altogether -- e.g. to air from wire, so you're able to reach warships. The further your recipient is from a radio emitter, the higher powered a radio signal you need to emit so it reaches its intended recipient, and thus the more far reaching the signal it is and the more prone it becomes to eavesdropping.
One small caveat: these signals were encrypted. Encryption then was nowhere near as strong as modern encryption methods (asymmetrical key exchange came later) but it was good enough that simply eavesdropping wasn't enough. (Aside, to give you a sense of the logistics involved: to work around encryption related hurdles, namely equipment and physical key exchanges, the US resorted to code talkers to exchange tactical messages.)