事(こと)なの

When the noun こと (or 事(こと) in its kanji form) is combined with なの, it conveys an explanatory meaning that is similar to what can be seen in the んです・のです grammar pattern. As with んです, ことなの transforms 'it is (A)', into 'it is that it is (A)', simply emphasizing the point.
ことなの may be seen in several different forms, and is quite often used in conjunction with というのは. In these types of sentences, というのは will present the topic, before ことなの will be used to show the conclusion that the speaker has drawn about that topic.
This expression may be used with any word in its attributive form, nouns followed by の, or な-Adjectives followed by な.

• 漫画家(まんがか)というのは漫画(まんが)を描(か)く人(ひと)のことなのだ。
  Manga artists are people that draw manga.
• 付属品(ふぞくひん)というのはメインの物(もの)に付属(ふぞく)している物(もの)の事(こと)なんだ。
  Accessories are things that come with the main product.
• 雨(あめ)が上(あ)がるというのは雨(あめ)が止(や)むことなのです。
  The thing which is 'for rain to rise' means 'to stop raining'.

という may also be used directly before ことなの. However, this is not always the case, and is even more unlikely when it was already utilized in the first half of the sentence.

• あの人(ひと)は超(ちょう)一流(いちりゅう)歌手(かしゅ)ということなの？
  Do you mean that person is the most skilled singer?
• 輸送(ゆそう)とは荷物(にもつ)を送(おく)ることなの。
  'Freight' means to ship a package.