The author's purpose is the reason why they wrote the piece that they wrote.  There can be many reasons why an author writes a specific work.  However, there are usually 3 categories for  why an author might write something:

 

To Persuade:  The author wants to convince the reader of a certain idea

 

To Inform: The author wants to give specific information to the reader

 

To Entertain: The author wants the reader to enjoy the written piece of work.