You 你 completed, have done 下了 some, a series of, 一番 difficult struggle 苦功 Together it's a statement that shows acknowledgement of someone's effort. It is a comment and at the same time a praise when used appropriately. A boss may look at some employee's work and say approvingly: "(Very well,) I can see you really have put a lot of effort into this project." Chinese people believe in hard work. To them, pain and constant struggle are necessarily an integral part of someone's success and survival. The ability to endure that hardship is considered as a virtue. Ex. 她很能吃苦 Therefore you should expect a smile when you say this to Jane: "Jane, I noticed your English has improved a lot! 你 (真的/的确) 下了一番苦功." Another example can be: "Jane, I like this painting of yours. 你 (真的/的确) 下了一番苦功." (Your painting skills really have improved a lot.) But try not to use this expression with your boss. Modern Chinese society is still quite hierarchical. Commenting is what superiors do to their subordinates, usually not the other way around. However, It is totally appropriate to use it with your friends or people whose 辈分 is lower than you.

The sentence means that "you made a great effort" or "you has worked hard". When we break down the sentence and see how it work, "一番" means "great" which is specially modified the word "苦功" (which means "effort"). Moreover, "下", which means "make", is a special verb for "苦功", and "下了" shows that the verb is a past tense. 你 下了 一番 苦功。 You made a great effort. Mind you, "一番" and "下" have so many different meanings in different situations. You should not use them as using "great" and "make". Here are some examples: 1. 今年利润比去年翻了一番。 The profit of this year doubled the one of last year. (In this sentence, 一番 means doubled.) 2. 他下楼了。 He went downstairs. （In this sentence, 下 means the verb "go" with a direction "down".）

I think it means that "you have made a great effort to do sth." while the result of your effort may be good or not.perhaps you achieve your goal,perhaps not. We can say ：“因为你之前下了一番苦功，所以你能顺利完成任务。”We can also say ：“这件事虽然没办成，但是我知道你确实下了一番苦功。（你还得继续努力）”

你下了一番苦工: means you do something diligently in some field(most of cases,It shows you succeed in the end by your efforts,but It also can hinder you fail)

I means that you have made a great effort to do something.Usually the result is good.