system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: In the movies when someone asks for a secure line, what exactly does that mean, and how do you do it?
gpt
You have a conversation over the phone between two people in different places.  The sound signals between these people is transmitted over the telecommunication network.  It's possible for someone else to be connected to that network and listen to the conversation these two people have.

A secure line is a protection against that.  It is done with encryption: the source phone will encode the sound signal so that it looks like gibberish, and the destination phone has the necessary key to decode the gibberish back into the original signal.  Anyone else intercepting the signal would only have the gibberish, but without the key they can't convert it back to sound.