Answer:
Helicopter blades act in the same way as the wings of an airplane. They generate lift with the air flowing faster on the top of the airfoil then on the bottom. When you reach a certain speed, depending on blade specifications and limits, the air flowing over the blades become equal on the top and bottom of the airfoil. This is what is known as blade stall. The blades no longer generate lift. The same effect in fixed wing aircraft is known as a speed stall. This concept also takes place when a helicopter flies too high. As the air thins with altitude eventually there is not enough air for the blade to "bite". Thus blade stall again.