According to The Stanford Encyclopedia of Philosophy, “Intentionality is the power of minds to be about, to represent, or to stand for, things, properties and states of affairs.”  Daniel Dennett, eminent philosopher of consciousness defines “the intentional stance” as a way to view the “behavior of a thing in terms of mental properties.”  From Dennett: “Here is how it works: first you decide to treat the object whose behavior is to be predicted as a rational agent; then you figure out what beliefs that agent ought to have, given its place in the world and its purpose. Then you figure out what desires it ought to have, on the same considerations, and finally you predict that this rational agent will act to further its goals in the light of its beliefs. A little practical reasoning from the chosen set of beliefs and desires will in most instances yield a decision about what the agent ought to do; that is what you predict the agent will do.” 
So, one can predict the actions of a ‘rational agent’ based on what one thinks the agent’s beliefs are and what one thinks its desires should be. If one is correct in this thinking, one should be successful in predicting the actions of the agent to satisfy its desires. In other words one assumes that the ‘rational agent’ has a theory of mind that one understands, if one is to make successful predictions. A theory of mind is: “the ability to attribute mental states to oneself and others and to understand that others have beliefs, desires, and intentions that are different from one’s own.” 
Autistic people have a condition called ‘Mind Blindness’, whereby they do not have the capability to attribute mental states to others. Surely, one can attribute mental states and desires to them; depending on the severity of the autism, one can predict their behavior under given conditions. They can be considered ‘rational agents’ in the sense that they have mental states themselves, even if they can’t attribute mental states to others.
What about profoundly retarded people, who cannot communicate verbally, like my brother? Many of the people who care for my brother attributed mental states or desires to him. I am not certain about their success in predicting his behavior. Perhaps all that motivates his actions are the signals from his stomach that indicate to himself that he is hungry, the signals from his skin indicating he is cold and other signals that indicate discomfort. Perhaps his non-essential desires are unknowable or non-existent and all his actions are conditioned responses. For instance, maybe the only thing he knows is that a dinner bell means ‘dinner’ and dimming of lights means, ‘bed time’.
The intentional stance is a way of thinking about something by mental states to it. As applied to people, one can predict another’s actions if one ‘read’ his or her mental states, desires and goals properly.
One can anthropomorphize inanimate objects as well, attributing mental states to them. For example, “My car is thirsty, better fill up the tank.”
Assuming an intentional stance toward my brother hasn’t helped me to understand him. Does he actually have mental states? If he does, suppose I never chose the right ones. What if he doesn’t?
 Jacob, Pierre, “Intentionality”, The Stanford Encyclopedia of Philosophy (Fall 2010 Edition), Edward N. Zalta (ed.), URL = <http://plato.stanford.edu/archives/fall2010/entries/intentionality/>
 Dennett, Daniel, The Intentional Stance, MIT Press, 1989 (p. 17)