how do 你 rate a pop 测试 question?

but 下一个 to the 问题 theres a bar that says 'easy, difficult,medium 'etc , so where does that come from?

Oh, that relates to what percentage of people got the 问题 right. The 问题 is "hard" if only a few people got it right, "medium" if the 答案 are spread, and "easy" if lots of people answered the 问题 correctly.