Dementor is a Dark creature, which feeds on human happiness and generates feelings of depression and despair. Professor Lupin Remus gave Harry lessons of Patronus Charm (Expecto Patronum), which is one of the most defensive spell against Dementors. The professor told him that Patronus is a kind of positive force, and for the wizard who can conjure one, it works something like a shield, with the Dementor feeding on it, rather than him. Patronus will work if you think of very happy, powerful memory
