What does prospero mean by "twelve year since, thy father was the duke of milan

By saying "twelve year since, thy father was the duke of milan", Prospero means that he himself who was Miranda's father, was a duke twelve years ago.

He explains that because Miranda can't remember how she got to the island. In fact, he is the only person she can remember, so she wanted to know about her past.

Besides, she begins to ask him about her past since he never told the full story to her.