The concept of "man-hour" was developed to describe the period of time that would be required in order to complete a certain task. So, let us say that translating a book requires 140 man-hours to be completed. That means that one man could do the entire translation in 17.5 days (working 8 hours per day), or 2 men could do the same job in 8.75 days (since both of them working together would complete the same amount of hours in less time). So, measuring tasks according to the "man-hour" concept allows to decided how many days the task will take to be completed depending on the number of persons working on it. I hope my explanation was helpful.