It means a person doesn't know how they will react until they are actually under fire ( actually experience a calamity ). . In combat this would be....being shot at. In everyday life this might be.....being homeless, or being divorced, or being told you have cancer, etc. This 'trial by fire ", how you react, reveals your true character.
A thorough test of someone's abilities or character under pressure.
It is a difficult test during one's lifetime, originating from 'trial by ordeal', an archaic method of determining one's guilt.