Autonomy is one of the basic concepts of importance to philosophical ethics. I will define it here as self-directedness. Here I will highlight a couple important background ideas before looking at autonomy specifically as relevant to ethics.
On absolute autonomy
At the outset, I think it is helpful to recognize that absolute autonomy is not possible for humans. Absolute autonomy would require that there be no constraints, physical or mental, upon the being that possessed it. If this were possible at all, then it would only be so for an all-powerful God. We might, for example — as many Medievals did — imagine that God has no constraints, that his wish is his reality. In alignment with such a view, the universe and our moral systems have taken their form precisely because God willed them one way and not another. Yet this view that God might be absolutely unconstrained also proved historically quite problematic. For, we might wonder — as many Medieval philosophers did — could God create a rock so big that he couldn’t destroy it? Or one so heavy he couldn’t lift it? If not, then God seems constrained by logic, by rationality. Such dilemmas, in consideration of moral questions, are found already in Plato’s dialogue the Euthyphro in which Plato asked whether something is pious because the gods will it or whether the gods will it because it is pious. Both the Medieval question and Plato’s question concern whether God is constrained by rationality. The dilemma, for Medievals, was that it appeared impossible for any entity or Being or the ground of Being itself to have logically contradictory essential properties or to be capable of logically contradictory feats. Not even God could square a circle. Those philosophers who did not maintain that human categorizations simply do not apply to God (and thus simply forfeit logical considerations) went on to argue instead that it appears even God is not absolutely unconstrained. He must be constrained by rationality or His own in-some-sense non-freely chosen nature.
Physical constraints on autonomy
Medieval metaphysics aside, we know of no creatures that are unconstrained, at least by physical and mental ability. We are constrained as singular spacial bodies not to be in two places at one time. A male is constrained, as male, not to bear a child. For humans, the set of activities that we are free to do have the initial constraints of our natural capacities. We can of course create technologies to expand our capacities, but none of these technologies remove all physical constraints. Beyond those limitations of our natural biological ability (including the capacities to develop technologies), though, there are other physical constraints. Of these, some will be self-imposed, some imposed by others. As an example of the former, a person may be theoretically free to walk along the beach in that she has legs and there are beaches. But if she wants daily beach walks but lives 2000 miles from a beach, then she will be unable to have them. She is only free to go somewhere closer to the beach if such walks are important enough.
Externally imposed constraints on autonomy
There are other activities that we have the physical capacity for but which we are prevented to do because of external powers. Someone who wants to take daily beach walks but is in a federal penitentiary in Colorado will be impeded from those beach walks, not because he doesn’t have legs or the will to move to the beach but because a policing authority is preventing him from realizing his abilities and desires. Other types of external constraints could come from financial or other needs: Someone may wish for those daily beach walks but not earn enough money to live near the beach or have the time to take the bus daily to do so because of the need to work to make a sustainable living.
Internally imposed (psychological) constraints on autonomy
Beyond such physical and external constraints on autonomy, there are also various internal constraints. Someone may wish to take daily walks on the beach but have agoraphobia, a fear of leaving the house. Such an individual might have a desire to go on daily beach walks but simply fear of doing so. Other internally imposed constraints may not be pathological, but simply a matter of having to prioritize among different desires. A person might desire on a given day to walk on the beach and to finish a novel but there is not time for both. So she has to decide which of these one wants to do. (Some cases, like the one I am calling externally imposed above of someone not being able to afford to live near the beach could be viewed as border cases. Such a person could theoretically quit her job, try to find a place to sleep on the beach, etc. I only bring this up as a theoretically minimally important distinction.) There are of course many types of internally imposed constraints. To turn to another example, a person may wish to be a doctor but (wrongly) believe that she doesn’t have the ability to pass medical school. Some psychological constraints can be pathological, like the agoraphobia mentioned. Others, like the lack of self-confidence and the lack of proper self-assessment, are not pathological even if they are unfortunate in that they can prevent individuals from pursuing some goals that they would objectively prefer and find more fulfilling. And individual constrained by false self-assessment like a falsely based lack of confidence prevent a person from autonomy.