What is a man hour?

A "man hour" is an hour of one person's time. If you work 8 hours a day you are working 8 man-hours. The difference between hours and man-hours becomes important when scheduling work for multiple people. If a project requires 100 man-hours then - in theory - if 10 people are allocated to do the work simultaneously, the project will be completed in 10 hours. 10 hours work done by 10 people gives you 100 man hours of work.