Moral and unit healing

Some buildings will raise your province morale.

To "heal" your units, place them in a higher morale province.

Along with a high morale province, use fortifications, they raise the morale of your unit depending on the level.

You can also use gold to boost the health of all units.

If your land units are in a province you own at daychange, they heal 7% of their missing HP. Air Units heal if they are on an Aircraft Factory or Airstrip that you own, but not on carriers. In both these cases, the land must be your own, not your coalition mates. Naval Units always heal 7% of missing HP. It has nothing to do with morale at all.