It is a place where players can show off the bosses they've killed. The display can be configured to show any display that a player possesses.
In order to display a boss lair, the player must have killed the boss once and must put in the jar dropped by that boss. In Vorkath's case, the player needs to defeat his post-quest variant.
The jar can be removed, returning the display case to its unfilled state and the jar to the player.