Helicopter blades, or rotors, are what keeps a helicopter in the air, and help it get from point A to point B. The turning of the main rotor creates lift, and tilting the main rotor moves the helicopter in any direction.
An interesting requirement for helicopters is that every helicopter must have at least two rotors. This is because the turning of a rotor creates torque in the direction opposing its rotation. If a helicopter had only one rotor, the rotor would spin one way, and the helicopter would spin the other way.
The second rotor spins in such a way that the torque it creates counters the torque of the first rotor, maintaining the stability of the helicopter and enabling easy control of the helicopter.