Okay, so imagine you're trying to talk to someone who speaks a different language than you. It can be really hard to understand each other, right? Well, a long time ago, some people decided they wanted to make a new language that everyone could use to talk to each other.
They called this new language Esperanto, and it was created to be really simple and easy to learn. People from all over the world started using it and they could finally talk to each other without any problems.
But over time, some people started to change the way Esperanto was spoken and written. They made it a little harder to understand and some people didn't like that. So they decided to go back and use the original version of Esperanto, which was called "Arcaicam Esperantom."
Basically, Arcaicam Esperantom is just the old way of speaking and writing Esperanto. People who use it think it's better because it's easier to understand and it's closer to the original language that was created. It's kind of like going back to how things used to be.