Refer to figure.
To counteract the longitudinal (pitch) stability of the main rotor many helicopters use a horizontal stabiliser. This is effectively a simple airfoil mounted near the tail. When the helicopter pitches up, then the tail pitches down.

This increases the angle of attack of the horizontal stabiliser, which as a result produces more lift. The increased lift at the tail provides a nose down pitching moment, which will counteract or overcome the angle of attack instability of the main rotor.

Thus, in forward flight, the horizontal stabiliser contributes to and improves the longitudinal static stability.

An increase in forward speed causes the rotor disc to flap back. This causes a nose-up pitch attitude. At the same time, the horizontal stabiliser will produce a nose down pitching moment.

The magnitude of the helicopter’s stability is dependent on the Centre-of-Gravity position and the distance of the horizontal and vertical stabiliser from the CG.

The stabilisers are only effective when the helicopter has a forward airspeed, otherwise no aerodynamic forces are produced on their surfaces.

The stabilisers are designed to give positive stability in forward flight and negative in rearward flight.