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, the Anti-Disapparition Jinx, which is a spell of the same kind but prevents people from Apparating out from an area.
- Pottermore (Mentioned only)