What are the three bar graphs showing?
They show Requisition, Power, and Waaagh! accumulated resources. It lets you see who is floating resources, or who is close to firing a nuke. Income rates are available on the other side.
What do the buttons up top do?
These are the telecast features and fog of war options. The down facing arrow is a UI arrow that can be used to highlight a squad, object, or a target on the ground. It places an arrow much like an objective arrow from the campaign. The cursor button is the "Large Cursor" button, and swaps the mouse cursor for a larger, webcast friendly mouse cursor. The "show infantry in FOW" button shows the enemy team underneath FoW.
All of these buttons are hotkeyed, so they are available when the panel is closed.
What does a referee do?
This was requested by some tournaments. Referees can pause the match, and have the ability to chat to all players. Normally observers only chat amongst themselves. The host of a custom game can designate any number of players as referees.
What type of games can we observe?
Observer is available in all custom games. It's up to the host to open up observers in their match.