"That" can introduce an explanatory dependent clause. "What" cannot. "That" indicates a specific quality of the object in question, or a subset of a larger set. "What" indicates the object in question itself, or.
"That" can introduce an explanatory dependent clause. "What" cannot. "That" indicates a specific quality of the object in question, or a subset of a larger set. "What" indicates the object in question itself, or.