Okay, so when people wrote down stories from a long time ago, they didn't have computers to make things easy. They had to write everything down by hand, which could sometimes take a long time.
One story that people wrote down was about Jesus, who was a very important person to a lot of people. Some people wrote this story down in different ways, which means that there are different versions of the story.
Now, in one version of the story, there is something called the "comma johanneum." This is just a fancy way of saying that there are some extra words in this version of the story that aren't in the other versions.
These extra words are about three things that people believe in - God the Father, God the Son (which is Jesus), and the Holy Spirit. But some people who study the story closely think that these extra words weren't actually part of the story when it was first written down.
So, to sum it up, the comma johanneum is just some extra words in one version of a story about Jesus, but some people think those words might not have been there originally.