Describe disorder in terms of entropy

Who is "they"? We can't tell you what someone means if we don't know who you're asking about.

If you're serious about understanding entropy, your first step should be to forget everything you've heard about "disorder" and start in on the real definition of entropy in terms of number of states.

 

What did that textbook say about entropy, disorder, and their connection to molecular motion? If we don't know what it is was trying to say, we cannot explain it to you. (This is one of the reasons that Physics Forums has a rule requiring that you provide sources)

I could guess that your textbook just said something about entropy representing disorder and left it at that. If that's your starting point, then we've already answered your question back in post #2 of this thread: Forget everything you've heard about "disorder" and start in on the real definition of entropy in terms of number of states. But now I'm just guessing what you're looking for, and if I guessed wrong that answer won't be helpful.

 

There is no single authority doing this, science is a much more open process.
"disorder" is not used in the scientific definition of entropy. You probably found some book that tries to explain physics to non-physicists, which means the science got simplified a lot - and we have no idea how exactly this was done in the book you read.

It is absolutely necessary.