Prevents Apparating into an area
The Anti-Apparition Charm is a spell that prevents people from Apparating into an area.  It was cast on Hogwarts School of Witchcraft and Wizardry long ago and can be lifted by the Headmaster of the time.
When student transportation to and from Hogwarts School of Witchcraft and Wizardry was unregulated, prior to the inception of the International Statute of Wizarding Secrecy in 1692, some students tried to Apparate into the Castle grounds, often with disastrous results because of this enchantment.
Behind the scenes
Another spell exists, Anti-Disapparition Jinx, which is a spell of the same kind but prevents people from Apparating out from an area.
- It was first mentioned in the Order of the Phoenix when Albus Dumbledore used it against the Death Eaters in the Department of Mysteries . That is how Lucius Malfoy was caught and sent to Azkaban.