Keeping the Dementors at bay at the moment.
“Dementors are among the foulest creatures that walk this earth. They infest the darkest, filthiest places, they glory in decay and despair, they drain peace, hope, and happiness out of the air around them… Get too near a Dementor and every good feeling, every happy memory will be sucked out of you. If it can, the Dementor will feed on you long enough to reduce you to something like itself… soulless and evil. You will be left with nothing but the worst experiences of your life.“
“A 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. In order for it to work, you need to think of a memory. Not just any memory, a very happy memory, a very powerful memory… Allow it to fill you up… lose yourself in it… then speak the incantation ‘Expecto Patronum.’“
(J.K. Rowling, “Harry Potter and the Prisoner of Azkaban”)