A terminal multiplexer is like a playground supervisor for your computer. Normally, you can only play one game at a time, like swing or slide or monkey bars. But with a terminal multiplexer, you can play lots of games all at once!
Think of each game as a program that you want to run on your computer. With a terminal multiplexer, you can have lots of programs running at the same time, on the same playground. When you want to play a game, you just switch over to that game and start playing.
It's kind of like having a bunch of TV channels all on one screen. You can switch between the channels and watch different shows, but they're all on the same TV.
A terminal multiplexer makes it possible to do lots of things on your computer without having to switch between different windows or tabs. It's like having a superpower that lets you do lots of things at the same time, without getting overwhelmed!