Newton's third law (for every action, there is an equal and opposite reaction) means that in every interaction (in this case, that of the objects in the image), there is a pair of forces acting on the two interacting objects. When you sit on a chair, your body exerts a downward force on the chair and the chair exerts an upward force on your body. There are two forces resulting from this interaction - a force on the chair and a force on your body. This is embodied in the image by the two objects exerting force on each other (indicated by the arrows).
Hope this helps.