Do you remember how in Jonathan Demme's Philadelphia (1993) Denzel Washington's character, an attorney Joe Miller, has the same request for everyone he is questioning: "Explain it to me like I am a four-year-old" (this is not the central quote of this post, by the way)? The phrase is used in a sarcastic, almost mocking, tone. Joe Miller implies that the subject matter is simple and obvious to everyone as is, but he wants to hear your version of it. Basically, it is an equivalent of "humor me" and it is a setup: you either going to admit his version of the truth or lie.
But the reality is that whatever it is you claim to know you actually should be able to break down into a comprehensive format digestible by, if not a four-year-old, but at the very least any layman. And if you cannot... here is the quote:
"If you can't explain it simply, you don't understand it well enough."