That setting allow the coupon to be used X times period. So if the same customer places 5 orders, and uses the coupon each time, that is the 5 "uses". If 5 different customers use it 1 time, its 5 "uses". So each time someone enters that code, the system will check to make sure there is still a "use" left. If not it will not allow that code to be used.
If you leave it blank, its like saying it has unlimited uses.