Endure pain, resist fear, outlast exhaustion, or hold together when everything is trying to make you stop.
Courage is what they write songs about. Resolve is what actually gets you to morning. It is an ugly quality — it does not look like anything from the outside. It is just the refusal to stop, maintained past the point where stopping would have been reasonable. Most men have courage for the first hour. A different kind of person is still moving at the start of the third day.
Resolve is the skill of endurance — holding together under sustained physical hardship, fear, torture, or the particular dread of something that is not quite human watching you from the dark. Make a Resolve (Vitality) Trial when the world is trying to make you stop and stopping would be the wrong choice. Resolve covers body and mind both — what keeps you marching, what stops the interrogator from cracking you, what lets you face the thing that smells like death and act anyway. For immediate physical feats of force, see Might (Brawn).
Examples: maintaining focus through a second sleepless night on watch; not breaking under questioning; facing something that triggers a deep-animal fear and acting anyway; keeping pace over three days of hard terrain without complaint.