A prefect is an honor awarded to specific 5th year students at Hogwarts School of Witchcraft and Wizardry. One male and one female student are chosen from each house. Percy Weasley is one when Harry first comes to Hogwarts, and in Harry, Ron, and Hermione's 5th year, Ron and Hermione get selected. Harry has an internal struggle against himself about why he didn't become a prefect, but he then realizes that he is happier without the extra burden.
The duties of a prefect include helping the 1st years find their way around the school, making sure everyone in their house behaves by the rules, and walking the halls to make sure there is no one who is being bullied and so forth. Also, in the Hogwarts Express at the beginning of each new year, they need to go to a meeting with all the other prefects to discuss the new year ahead of them. Prefects also help decorate the school for the holidays.