The Dementor's Kiss is a fictional term from the Harry Potter Universe.
The Dementor's Kiss is a punishment given only to those who have committed the worst kinds of wizarding atrocities. The Dementor is known for draining the happiness out of every one it has a close proximity with, but if it kisses a person, it sucks away their very soul, leaving them empty, emotionless husks of their former selves.
In Harry Potter and the Prisoner of Azkaban, Sirius Black is attacked by Dementors on the shores of the Lake in the Hogwarts grounds, but is saved by Harry using the Patronus charm. Later, before he and Hermione use the Time Turners for the first time, Sirius is imprisoned and subjected to the Dementor's Kiss, as the ultimate punishment for his escape and the crime he did not commit.