Just like "ask me anything" does not necessarily mean anything, it's an expression at this point.
The ELI5 subreddit describes it in its rules as such:
"Rule 4: Explain for Laypeople
As mentioned in the mission statement, ELI5 is not meant for literal 5-year-olds. Your explanation should be appropriate for laypeople. That is, people who are not professionals in that area. For example, a question about rocket science should be understandable by people who are not rocket scientists."
The ELI5 subreddit describes it in its rules as such:
"Rule 4: Explain for Laypeople
As mentioned in the mission statement, ELI5 is not meant for literal 5-year-olds. Your explanation should be appropriate for laypeople. That is, people who are not professionals in that area. For example, a question about rocket science should be understandable by people who are not rocket scientists."