Used to prevent Disapparition and/or Apparition in an area for a period. Presumably can be used to prevent an enemy from entering a defended area, or used to trap an enemy in an area.
Mentioned in Order of the Phoenix, used by Dumbledore to trap several Death Eaters in the Department of Mysteries.[OP Ch.36] Also cast long ago on Hogwarts, the reason why, as Hermione quotes often throughout the series, "no one can Apparate or Disapparate inside the Hogwarts grounds." In Deathly Hallows, it prevented the trio from escaping Hogsmeade when Death Eaters placed the same spell around the area.