From Populous Wiki
Repairing is repairing damaged that has been done to your base. It is important to repair your base when an enemy has done some heavy spells on you. Earthquakes can shape your land in nasty ways, to repair use Landbridge to make the land level again. Some of your defence may be knocked out so repair your towers, build more and train new followers. Just remember to keep your base how it needs to be otherwise it will only get weaker!
Sometimes you don't have to fix the land. The enemy might have cast a Landbridge to a hill, but you can defend on it by laying a tower or some troops, it blocks the weak point to your base. Lots of firewarriors, warriors can protect the hill. Towers may be good in case of firestorms, and use braves or even ghosts to keep enemy shamans busy.
You may have had a lot of damage on your home buildings, don't give up just send a few survival braves to repair your huts and you will be making mana again like you were, plus new people again.
Damaged buildings need around 90 seconds to become apt for rebuilding again. This means it's a waste of population to send braves to repair buildings before the 90 seconds are over, as they will not actually do anything. After 90 seconds, units will search for wood and re-mantle the building.