What does the novel say about "courage" essentially is?

I think that Henry learns that courage is not simply winning in battle. Henry realizes that there are no heroes in battle and that, in some form, everyone loses. Henry certainly becomes a mature soldier by the end of the book. This maturity is really the courage Crane is alluding to. Maturity is the ability to see things for what they are. Fear, anger, empathy and instinct are all part of what makes a soldier "courageous".