"Explain like I am five" is a saying, not the rules of the subreddit. It's an expression that means "explain this to me simply so I understand." The rules of the subreddit are that we need to explain concepts for the actual people posting on Reddit who are asking questions, not a theoretical five-year-old.

There is a subreddit about explain like I am an actual 5-year-old for people who want to do that sort of challenge, but the rules of this sub are that we're going to answer questions simply, but not literally for 5 year olds.