The Asgardian Shield was a large, dome-shaped energy shield used to defend the central palace of Asgard.
The Asgardian Shield was activated by Heimdall during the Sacking of Asgard to protect the palace from an invasion by the Dark Elves. The generator was destroyed by Algrim to allow the Dark Elves access to the Hall of Asgard.
After the battle, Fandral let Odin know that it would take some time to repair the shield, leaving Asgard vulnerable if Malekith chose to attack again. This information caused Odin to order the closure of the Bifrost Bridge, and, ultimately, forced Thor to devise a plan for removing Jane Foster and the Aether from Asgard by alternative means.