To understand what exactly Dementors are we must know their origin. A long time ago, Dementors were chosen by the Ministry of Magic to guard the prison of Azkaban. Those soulless non-beings were the perfect way to take care of prisoners. A guardian that cannot be defeated and cannot be bribed. At least it is what they thought. Lord Voldemort found a way to control them. He promised to give them many souls to feed on, many happy memories to suck out of humans, many people to turn into empty vessel. The Dementors helped Voldemort get into the ministry of magic. After that incident Dementors could no longer be trusted. They were banished from Azkaban and left to wander the world.
Nowadays they seek new souls to suck on. Young wizards are taught how to defend against them because they present a big threat for every human being. But how exactly can you defend yourself against them when they cannot be harmed? Their strength is based on sadness and despair, you can reduce their power by confronting them with a powerful memory, with the happiest memory you have. A spell called Patronus Charm uses your memory to summon a Patronus – a spirit-like animal made of pure hapiness and hope. It weakens Dementors forcing them to flee.
The Patronus Charm is one of the hardest spells to learn. It can take many tries to learn which leads to death of many young students. In a contact with Dementor you are only given one chance. Fortunately teachers in Hogwarts can help you learn the Patronus Charm and if anything goes wrong they can protect you.
Make sure you know how to handle Dementors before you head out for your adventure so it won’t end too early in a such brutal fashion.