Boom stay

From RC Helicopter Wiki

The boom stays run from the bottom rear of the helicopter frame up to a common point on the boom near the tail rotor (often at the horizontal stabilizer).

The tail stays help to stiffen the tail section, both horizontally and vertically. This allows the tail rotor and gyro to work more accurately, and also helps reduce vibration from the tail rotor, by shortening the area that can vibrate sympathetically.