According to Wikipedia
“Moral agency is a person's capacity for making moral judgments and taking actions that comport with morality.”
This seems straightforward. An adult is usually considered a moral agent. You as an adult are responsible for what you decide to do. If you rob a liquor store, or rush into a burning house to save a baby, or cruise the internet during office hours when you’re supposed to be working, you are responsible for your actions. You decided to do it. You did it.
A moral agent is responsible for his or her reactions to things. For instance, if your boss fails to promote you and you therefore decide to shoot him, you cannot claim that you are not responsible for the shooting. You can’t say: “Hey, he failed to promote me. I was only reacting to his action. You have to blame my late boss for this!” The concept of moral agency demands that no matter what the other guy does, you are responsible for what you yourself do.