When a "force[s] act on an object" it is discussing how a force is influencing an object.
For example: A box that is being held up stationary in the air with a rope has force acting on it. There is the force of gravity, which wants to push the box downward. There is also the tension force, which is the rope that is holding the box up and keeping it in the air.
Since the box is stationary, both the tension force and the force of gravity must cancel out.