One way of classifying clauses is according to the type of ‘job’ they do in clause/sentence structure: adverbial clauses do the same job as adverbials, adjective clauses the same job as adjectives, and noun clauses, the same job as nouns and noun phrases.
There are several types of noun clauses. One type, the relative noun clause typically begins with the word "what"—or some other "question word" such as "who," "why," or "when." For example:
• Jack told Jill what she wanted to hear.
• What started all the trouble between Jack and Jill was his mother's visit.
In the first of these sentences, the relative noun clause is the object; in the second, it’s the subject.
The noun clauses in the two above sentences are "nominal" relative noun clauses: they refer to (or "name") a real, thing or situation (Jack's mother's visit for example.)
A slightly different type of relative noun clause, the "interrogative" relative noun claus is actually more common. These clauses refer not to a real situation but to a question or a lack of information. For example:
• Jill asked Jack what time it was.
• Jack had been wondering where Jill was.
• Jack's mother was often confused about where she was.
Non-native speakers often make the mistake of using "question order" (verb before subject) with interrogative relative noun clauses. For example:
• Jill asked Jack what time was it.