Asgard is one of the Nine Realms where immortal Asgardian gods and godesses reside. The kingdom was ruled by Odin, the father of Thor the Thunderer and Loki the Trickster.
Asgard has always been attacked by evil forces from the realms intending to take over while Odin went into the Odinsleep to replenishes his OdinForce powers but were stopped by Thor.
The Asgardian kingdom was conquered by Loki and his allies while Thor remains on Midgard (Earth) because of Loki's releasing captured supervillians from their prisons just to distract him.
The Asgardians who were captured or remained hidden rebelled against Loki's rule thanks to the unlikely arrival of the band mortal heroes from Midgard known as the Avengers. Together, they defeated Loki and trust was achieved between mortal heroes and Asgardians.