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.