The Patronus Charm

Expecto Patronum

The patronus charm is used to defend the dementors.If you peformed correctly a silvery animal will be seen. 'Expecto Patronum' is one of the famous charn in movie.It's used by Harry Potter to save his future version. And you can see his silvery animal,a doe.

Harry Potter and the Deadly Hollows
Harry Potter and the Prisoner of Azkaban