It's used with nouns and　 な　adjectives. Compare: 難しい　= it's difficult 難しいの　= it's a difficult one 簡単だ　= it's easy 簡単なの　= it's an easy one

It is an informal, shortened form of 「なのです」or 「なのですか?」. 「あなたは台湾人なの?」　("Are you Taiwanese?" ) 「いいえ、わたしは日本人なの。」("No, I am Japanese.")