This new update has been added with determine whether something is a top story based on lots of factors, include the relationship with the person , how many comments and like it has , the type of the story , and others. For example : a friend updated a status that may be not the top story without many friends comment on it. Simply you can say the more comments the more views the more attractive and it will turns into top story.
If you are disconnected with Facebook for a long while when you come back and check your news feeds you will able to see all the top stories at the top of your news feeds.
If something is not interesting for you and you won't like to keep it as your top story in the right side you will able to see a an arrow click on it unmark as top story from the menu and you are done. Now you will not see that story as a top story in your news feeds.
I hope that explained you what is a top story. but in simple word - a post that has more reviews becomes top story.