A force acts on an object that contains some matter. There is a gravitational field of earth as we know. It is present all over on Earth even in air or vacuum near earth. But this gravitation attraction force is experienced only by an object. A volume of vacuum does not experience any gravity force near earth. This is the meaning of the statement. When there are multiple influences, impacts, pushes, pulls, pressures, act on an object we have multiple forces acting on that object.
A force acting on an object causes the body to change its speed and direction. It could cause a change in shape of the body. A set of forces acting on a body may result in a changed speed & direction of the body or may balance themselves to keep the body to remain stationary. FOr example forces of gravity and a force called normal reaction of the floor act on a pen lying on the floor.