Answer:
The Third-Floor corridor hides a trapdoor leading to the Underground Chambers, guarded by Fluffy. Harry, Ron, Hermione and Neville first visit the corridor during Harry Potter and the Philosopher's Stone, when they are running from Filch. This is where Fluffy, the three-headed dog, guards the entrance to the underground chambers protecting the Philosopher's Stone.
Step-by-step explanation:
The Third-Floor Corridor, known as the Forbidden Corridor in Harry Potter and the Philosopher's Stone, is a corridor on Hogwarts' third floor. At the start of the year, students are warned to stay away from it if they do not wish to die a most painful death. The Charms Corridor leads to this corridor.
Evidence from the book:
Chapter 9: The Midnight Duel
"The third-floor corridor is out of bounds to students during Harry's first year. Dumbledore warns that anyone who does not wish to die a painful death ought to stay away. This is where Fluffy, the three-headed dog, guards the entrance to the underground chambers protecting the Philosopher's Stone."
~Philosopher's Stone, by J. K. Rowling