The sidebar is the pop-up of information that is displayed on the right hand side of the user's screen by default. This is helpful for showing a lot of player specific information all in one location. The sidebar settings that you choose are persistent, meaning the server will remember your choice when you log back in.
/sb - Enables the sidebar, or shows the default sidebar if another one is already enabled.
/sb off - Disables the sidebar. This will remove any instance of a Sidebar from the user's screen.
/sb quests - Enables the Questing Sidebar. This will show information relating to 4 quests. If you have more than 4 enabled, it will show the 4 most recently interacted with Quests.