This hook sentence provides the meaning of an important word involved in the main idea / topic / theme. 

Example 

if you are writing an essay arguing Iago succeeds by luck more than skill in Othello, you could use this:

“Irony is a literary technique where events or actions are deliberately opposite to what one expects.”

Even though the word “irony” is often used by people, the meaning of it might not be fully understood by everyone. Words may be challenging or confusing and not always fully understood by people, so giving the reader a clear meaning of that word will better help them follow your essay.