I could be wrong on this,but i believe "combat time" was the way to make sure certain units -like aircraft- had some downtime.
So you couldnt have the same aircraft in the air 24/7.
I think it represented fatuge(sp) of the men and equipment.
Actually, tinjaw got it right. It's the number of hours they can fight before needing to get resupplied. This became necessary to compare units since a unit that has an attack of 20 with 4 combat hours is better than an attack 30 with 2 combat hours in certain circumstances.