In physics, the tension in a rope is the pulling force exerted by it on another object. The tension in a rope is equal at all points in the rope and its magnitude is determined by the forces acting on the rope.
The total weight of the performer and the unicycle on a tight rope is equal to 155 pounds. Weight is a force acting downwards on the rope. In this case, the rope is pulled down by a force equal to 155 pounds. This creates a tension in the rope equal to 155 pounds. The tension is not being exerted on the rope by the unicyclist, it is a force created in the rope that prevents it from ripping apart. If the tension created in the rope was larger than what the material that the rope is made of could withstand it would break.