(Known in psychological circles as Neuroticism)
High scores = secure/confident
Low scores = sensitive/nervous
People that score high in stress tolerance (or inversely low in neuroticism), aren't easy to upset and are less emotionally reactive to situations. They tend to be calm and collected in the face of threatening situations.
Anger, depression, and generally negative emotions are the calling card of low stress tolerance. People that score low in stress tolerance tend to see ordinary situations as threatening, and minor inconveniences as extremely difficult. The term “Blowing things out of proportion” was more than likely coined by someone dealing with a low stress tolerance person.