According to Kant, a moral agent is autonomous. This means that they are not subject to control by anyone else. Essentially, they live by a moral law which they have given to themselves—a rational law which must be capable of being applied to all people in all possible circumstances. This doesn't mean that the moral subject is a law unto himself in the normal sense of the term: that he can do whatever he wants, whenever he wants. It means that each moral subject has freely chosen to act according to the rational demands of the moral law that is within each and every one of us. As the moral law is rational, it is also universalizable. In other words, the contents of the moral law must be capable of being applied universally, and clearly that wouldn't be possible with selfish actions; if universally applied, they would lead to chaos and anarchy, not the moral society of autonomous individuals that Kant recommends.

In the philosophy of Immanuel Kant, a moral agent is a being that is in control of its own choices.  It is something that is autonomous (at least with regard to its moral choices) -- nothing can compel it to act in any given way.  The moral agent is able to determine for itself whether it wants to act badly or well.

According to Kant, moral actors are not simply controlled by their desires (at least not necessarily).  They are not animals (non-human animals, that is) that are driven by instinct.  Instead, they have a rational will that allows them to make moral decisions for themselves.