A Patronus is a fictional magical guide animal that appears in the children’s book series, Harry Potter by JK Rowling. Each magical person in the book has a different patronus that takes the form of an etherial animal. 

Patronus is a Latin word that can mean guardian or protector.

It could also be in reference to the movie "Harry Potter." In the movie, the main character Harry uses the phrase "Expecto Patronum" to summon a Patronus and cast a defensive spell against Dementors.

It is not a word that is worth committing to memory if you want to speak English conversationally (outside of conversations about Harry Potter ;-)  I have never used this word, not once.  

Watch Harry Potter and the Prisoner of Azakaban  and you will know it.  You are not gonna regret it, is a great movie and the books are also amazing! :) 

I don't know I've heard that somewhere, and i want to know what does it mean..