This hook sentence is a short story, one or two sentences, that is usually about a real incident or person. Anecdotes are often used when you are writing an essay about a specific incident, person or character. The anecdote can also be something interesting about the author of the work (novel, play or poem). 

Example

If you are writing an essay about the topic of oppression in Animal Farm, then you could:

Use the story of George Orwell absolutely hating prep school due to the “all-powerful monsters” who operated it. 

Anecdotes nicely connect the story to the reader making them more interested in your essay and possibly swayed to your point of view.