Which Medical Condition Causes Wheat Intolerance?

I just learn that one of my cousins cannot eat baked goods that use plain flour but can eat with spelt flour(forgive me if I spelled it wrong).
I checked what it is but I do not understand his situation (he did not explain it clearly to me)
It was something about enzymes not being able to break down regular flour (the same for those who cannot digest alcohol).
I do not want to use ingredients that could hurt someone. Is there a condition I should be aware of? It is something unheard of for me and I am very worried about him.