Okay kiddo, you know how sometimes your teacher gives you homework to write a paper about something you learned in class? Well, imagine if someone offered to do that homework for you, but they sold that same paper to lots of other kids too. That's what a research paper mill is.
They're websites where you can buy pre-written papers on all sorts of topics. You don't have to do any of the work yourself, you just pay them money and they give you a paper to turn in to your teacher.
The problem with these paper mills is that they usually copy information from other sources and put it together to make a paper, instead of doing the research and writing themselves. This means that the information in the papers can be wrong or even plagiarized, which is like copying someone else's work without permission.
Using a research paper mill is like cheating in school, because you're not doing the work yourself and you're not learning anything from it. It's important to always try your best and do your own work, even if it's hard. That's how you learn and grow!