What is the concept behind being able to continuously pour iced tea while rolling an airplane?
The maneuver Bob Hoover is performing is a specific kind of roll called a barrel roll.  If you look at the path traced by this maneuver it is a corkscrew rather than a linear path that you might be imagining.  Because the airplane is rotating about a longitudinal axis as it translates forward there is centripetal acceleration toward the axis of rotation, or "up" in the reference frame of the person in the airplane.  

To the observer in the reference frame of the airplane (e.g. the pilot or a passenger), the centripetal acceleration is interpreted as the fictitious centrifugal force.  This is exactly the same mechanism that you experience when driving around tight curves, though the force is in a different direction.  The end result in the airplane is that for a properly executed barrel roll, the centripetal acceleration causes the net force you experience inside the airplane to remain constant throughout the roll (in your reference frame, with the net force pointed "down").  

If you were to close your eyes you would not know the plane rolled.  Likewise, the iced tea does not "know" it is upside down.  For a less dramatic demonstration, next time you are on a commercial airplane pay attention to when the plane is banking and turning and note that a cup of water on the tray doesn't notice the turn.  The cup stays in place and the fluid remains level (with reference to the airplane).  

You can also demonstrate this with a small bucket and some rope.  Put some water in the bucket, tie the rope to the bucket and swing it in a circle vertically (be careful).  The water stays in the bucket if you swing it fast enough, and this is the same mechanism you see in Bob Hoover's video except it is the whole airplane being swung around in a circle like the bucket.