Imagine you have a really long string, and you want to talk to your friend who's at the other end of the string. You start speaking into the string, and the vibrations from your voice travel down the string to your friend's end. Your friend can hear what you're saying and can respond by talking into the string, and their voice vibrations travel back to you.
In telecommunications, a long line is a bit like that string - it's a really long wire that stretches for miles and miles. But instead of carrying your voice, it carries electrical signals that allow people to talk to each other over the phone or internet.
The long line is made up of lots of smaller wires, which are wrapped together to make a thick cable. These cables are laid underground, strung up on telephone poles, or run across the bottom of the ocean (like the transatlantic cables that allow people in America to talk to people in Europe).
The signals that travel down the long line are made up of 0s and 1s - just like the language of computers. These signals are sent from one place (like your home or cellphone) to another (like the network of wires that make up the internet).
When you use your phone or computer to make a call or surf the web, your signal travels along the long line until it reaches its destination. And just like with the string example, the person on the other end can hear and respond to your message.
So, to sum it up, a long line in telecommunications is a really long wire that carries signals between people and devices, kind of like a really long string that allows you to talk to someone far away.