This only applies if your heli has a tail belt, obviously.
The tension of the tail belt should be set according to your flying style:
For beginners and light sport flying, the tail belt tension should be firm but not loose. This will decrease friction and increase your flight time and tail belt life.
For aerobatic flying, the tail belt tension should be somewhat tighter. This will increase friction, decrease flight time and tail belt life, but will prevent the tail from slipping during hard maneuvers.