Reddit glossary
What is ELI5 on Reddit?
ELI5 stands for explain like I'm five, a request for a very simple explanation.
There is a whole community for it, r/explainlikeimfive. In other places, people just write it in a comment to ask for a plain answer.
If you are promoting something
It is a good reminder for your own writing. If you can explain what you sell in a sentence a ten-year-old could follow, you are doing it right.