The concept is simple, restrict your vocabulary to the 1000 most common words. If you can explain something using this reduced vocabulary, then you really understand the topic. This is a decent test of understanding because it’s easy to learn a word, and even use that word in the right context, but still have no idea how it relates to other things.
This is not to say that specialized vocabulary is bad, it’s quite useful for communicating between people with a lot of shared context. But when explaining things to people without that context, it’s important to be able to expand jargon words into common words, even if it results in a larger, slightly less accurate version.
Here is an animation of the editor mode in action: