When an aircraft spins lengthwise (along its longitudinal axis) it’s called a roll. Not all rolls are the same. An aileron roll means you use the flight controls on the wings–the ailerons–to roll the airplane.
With this maneuver the airplane experiences both positive and negative G. The negative G occurs when the aircraft is upside down. You could not pour a glass of water during this maneuver.
Another kind of roll is called a barrel roll. In this maneuver the pilot applies the control input to cause an aileron roll while at the same time applying up elevator, which causes the aircraft to rotate around its lateral axis (makes the nose go up or down from the pilot’s point of view).
Notice in the second video the nose goes both above and below the horizon as the aircraft rolls. The up elevator is constantly making the nose come up from the pilot’s point of view, but since the aircraft is rolling the nose goes up, above the horizon when the aircraft is upright, then below the horizon when the aircraft is upside down (inverted).
When you apply up elevator and cause the nose to rise (from the pilot’s point of view) you are changing the flight path of the aircraft. With no elevator input the aircraft travels straight. When the pilot applies up elevator the aircraft begins to travel in a curve, with the bottom of the aircraft to the outside of the curve. The plane–and everything in it–is trying to continue moving in the direction it was already moving, but the up elevator is forcing everything to move in a curve. In simple terms, if up elevator is applied and held from straight and level flight the airplane’s flight path will eventually curve up and over in a classic loop.
When a pilot performs a barrel roll he simply applies up elevator during an aileron roll, keeping the G load steady throughout the maneuver. Everything inside the airplane is being forced toward the bottom of the plane as a result of the up elevator. The roll is slow and steady enough so that the positive G from the up elevator is the overriding force on the aircraft and everything inside it. It’s exactly the same as when you swing a bucket with water at the end of a rope. The water stays in the bucket because of the increased G load resulting from the centrifugal force.