Intense, extreme, to the max. The phrase comes from olde-fashioned steam engines; basically, they had two weighted balls attached to a vertical shaft which was connected to the engine. As the engine speed increased, these two balls would be raised higher because of centrifugal force! At top speed, they would be parallel to the ground, nearly touching the "walls" of the shaft! Hence, "balls to the wall."
"Balls to the wall" is a phrase with an interesting origin. Remember, when it comes to etymologies, the best story wins!