Time field

The Time Shield was a mysterious crack in the fabric of time and space caused by an unidentified explosion. It had the ability to wipe individuals out of time itself and remove events from history, though time-travellers still had the ability to remember them. The Shield can also close and subsequently travel to another time. The closed Shield can be opened by a Sonic Screwdriver. The Shield seemingly erased the CyberKing (DW: The Next Doctor) and the Dalek invasions of 2007 (DW: Doomsday) and 2009. (DW: The Stolen Earth/Journey's End)

One in Amy Pond's bedroom connected it to an Atraxi prison in 1996. (DW: The Eleventh Hour) Later appearances of the Shield were on the Starship UK in the 33rd century. (DW: The Beast Below) and the Cabinet War Room in 1941. (DW: Victory of the Daleks) The Shield later opened to an incredibly large size aboard the Byzantium in the 51st century. It erased Christian, Marco, Pedro, Phillip and the Weeping Angels from existence and subsequently closed. The Weeping Angels themselves had feared it, and attempted to escape. (DW: Flesh and Stone)

For an unknown reason, unbeknown to Amy until her encounter with the Weeping Angels, it was following her everywhere she went with the Eleventh Doctor, appearing wherever she went after she arrived.

Behind the Scenes
The Shield was inspired by a crack Steven Moffatt saw in his son's bedroom. Moffat has confirmed that the Shield will appear in all episodes.