The Anti-Apparition Charm (incantation unknown) 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 only 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.
The Horcrux cave was presumably enchanted with this jinx, as Harry Potter and Albus Dumbledore were forced to approach it from a distance and overcome Lord Voldemort's enchantments to enter, similarly, one could not Disapparate in order to get out.
Behind the scenes
- Another spell exists, the Anti-Disapparition Jinx, which is a spell of the same kind but prevents people from Disapparating out from an area.