a doer: the person who actually carries out what has been said to them, whether a message heard or a law given, instead of only taking it in
Greek builds this noun on the ordinary verb "to do" or "to make" (ποιέω), and the ending names the person who performs the action. Everyday Greek used it for a maker, and most often for a maker of verse. Jewish writers working in Greek pulled it in a different direction: the person who carries out what God has said. That is the sense the New Testament almost always wants.
It never floats free. Something is always being done: the message, the deed, the law. And it almost always arrives paired with its opposite, the hearer (ἀκροατής). Romans 2:13 says the hearers of the law are not the ones God counts as righteous, the doers are. James 1:22 tells his readers to become doers of the message and not hearers only, then spends the next few sentences on a man who glances in a mirror and walks away having forgotten what he looks like.
James 4:11 swaps the pairing. There the contrast is not doer against hearer but doer against judge (κριτής): a person who runs down a fellow believer has quietly climbed above the law and started ruling on it rather than keeping it. The point holds either way. The law is something you carry out, not something you sit over or merely listen to.
The choice of a person-word matters. Greek could just as easily have used the verb and said "you did what the law says." Naming a kind of person instead turns it into a description of character, what someone habitually is, rather than a record of something they once managed.