A FlyBarLess (FBL) helicopter is a helicopter without a flybar to help stabilize it. Since the mechanical flybar is absent, its functions have to be taken over by a flybarless control system, which is sometimes called a virtual flybar. It consists of a series of gyros and software controlled mixers.
A flybarless system works very similar to a heading hold tail gyro, but it works on all axes. The helicopter will keep is current orientation, mostly unaffected by outside disturbances (eg. wind). The heli will only changes it's orientation when commanded to do so by the pilot. Most pilots describe this by saying the heliopter "tracks better", thus it needs less constant corrections.
The benefits of a flybarless system are:
Helicopter rotor systems having more than two blades are all flybarless. Flybarless rotor heads with more than two blades are very useful for scale models. While it is possible to fly a flybarless head without these extra gyros and mixers, it is very difficult.