Even though this skill should only reduce damage by up to 11%, it has been found through testing that if you invest even one point, Burn damage from DoT effects are reduced by ~50%, with a ~4% reduction per point afterwards.
"Sterner Stuff" is an idiom coined by William Shakespeare, classically notated in this quote from his play Julius Caesar: "When that the poor have cried, Caesar hath wept; Ambition should be made of sterner stuff..."
It could be a reference to Optimus Prime's incredulous response to Megatron's ploy to stall his actions in the original, 1986 Transformers movie. "You, who are without mercy, now plead for it? I thought you were made of sterner stuff."