"What my arguments are about is simple" is a response sentence to a question. This question is presumably "What are your arguments about?". In this case, 'what' is tricky because it is playing the role of a determinant without fully answering the question. Generally, 'what' is a question determiner and replaces the subject. ex. "What is x?" In this case, as a non-question answer, 'what' is playing the role of only a determiner, similar to "this" or "the", and the subject is not a singular noun, but a noun phrase "my arguments". Here's the tricky part: in this style of phrasing, 'what' is playing two parts. It is referring to both 'simple' and 'my arguments are'. ie. "My arguments are what?"; "What is simple?" "What is simple" is the encompassing structure that embeds the question "What are my arguments?". What is simple? My arguments are simple. However, by adding the preposition 'about' at the end of the noun phrase 'my arguments' disrupts the answer, because the answer is supposed to be a noun, not an adjective. Thus, the style of the phrasing make it so the 'what' is reiterated to emphasize the answer: What --- (my arguments are about) --- is simple. Because the response to the 'about' preposition is to be a noun, this sentence thus begins a paragraph response, since more explanatory sentences are supposed to follow. Hope this helps! I know it's a lot of grammar technicalities.... x_x