Answer:
In Chapter 5 of "Harry Potter and the Prisoner of Azkaban," titled "The Dementor," Harry Potter is traveling on the Hogwarts Express to Hogwarts School of Witchcraft and Wizardry for his third year. During the journey, the train makes its usual stops at various stations along the way.
At one point, the train passes through a particularly dark and desolate stretch of land. As it does so, a Dementor boards the train. Dementors are dark, hooded creatures known for their terrifying appearance and their ability to drain happiness from those in their presence. They guard the wizarding prison, Azkaban, and are feared by both magical beings and Muggles alike.
The reason the Dementor stops the train is that it senses the presence of Sirius Black, a dangerous escaped prisoner from Azkaban, who is believed to be seeking Harry Potter. Dementors have a strong affinity for locating their targets by sensing the despair and negative emotions associated with them. Sirius Black is thought to have betrayed Harry's parents, leading to their tragic deaths, and is now after Harry.
When Harry encounters the Dementor on the train, he experiences an overwhelming sense of fear and despair. The Dementor's presence triggers his most traumatic memories, particularly those surrounding the death of his parents. These powerful emotions cause Harry to lose consciousness and fall into a state of near unconsciousness, reliving the night of his parents' murder through flashes and sensations.
Thankfully, Harry is saved by the timely intervention of Professor Remus Lupin, the new Defense Against the Dark Arts teacher at Hogwarts, who repels the Dementor with a powerful Patronus charm. A Patronus is a magical defense against Dementors, taking the form of a silvery, animal-shaped guardian. Lupin's Patronus takes the shape of a wolf.
Lupin's arrival marks the beginning of a significant connection between the two characters, as he becomes a mentor to Harry and helps him learn the Patronus charm. The Patronus charm becomes an essential tool for Harry in facing Dementors throughout the rest of the series, symbolizing the power of hope and positive emotions in the face of darkness and despair.