自然秩序意味着什么在这个上下文中？

I have a question which appeared in a past paper (I'm revising for my exams) and I came across this word natural order which appears to be a keywords since it was written in bold on the paper. I've looked online at Natural Order but I couldn't find anything that related it to arraylist's like my question asks.I have a question which appeared in a past pape