The tension in the belt which is passing over the two pulleys (i.e driver and follower) when the pulleys are stationary is known as initial tension in the belt.
When power is transmitted from one shaft to another shaft with the help of the belt, passing over the two pulleys, which are keyed, to the driver and driven shafts, there should be firm grip between the pulleys and belt. When the pulleys are stationary, this firm grip is increased, by tightening the two ends of the belt. Hence the belt is subjected to some tension. This tension is known as initial tension in the belt.
To = initial tension in the belt
T1 = Tension in the tight side
T2 = Tension in the slack side
To = (T1 + T2)/2