The Restricted Section is a part of the Hogwarts Library which is off limits to students unless they have written permission from a teacher.
From the Story[]
"The Restricted Section is an area located at the back of the Hogwarts’ library. Separated by a rope, students need a signed note from a teacher to enter. It contains information on powerful Dark Magic (usually only studied by older students working on advanced Defence Against the Dark Arts work).
"Harry enters this section during the Christmas holidays to search for information about Nicolas Flamel; he accesses the area without permission, using his Invisibility Cloak to get in."
Discovered in Book 2, Chapter 10, The Rogue Bludger[]
"The Restricted Section is an area located at the back of the Hogwarts library. Separated by a rope, students need a signed note from a teacher to take books out of the Restricted Section. It contains information on powerful dark magic (usually only consulted by older students working on advanced Defence Against the Dark Arts work).
A copy of Moste Potente Potions is kept in the Restricted Section, along with other Dark and dangerous books."