There are several things that can affect your mercenaries' morale outside of combat, such as spending more on wages/upkeep and building various upgrades.
As far as I can tell, morale in a mission is a percentage that fills up and allows access to some activated skills and a passive bonus when over 50%.
What does it main to gain +1 morale? Does this increase my starting morale in a mission, or make it accumulate faster? Or is out-of-mission morale a completely different thing that affects something else entirely?