Throughout the novel, Roger's actions show him to be a boy who is the first to break with civilization. He is the first to throw rocks near a lilun, the first to stab a piglet, and the first to take a cheap shot against Piggy. The nameless authority that Roger wields is violence. He has no qualms about picking on the smaller children and torturing Sam'n'Eric to get the knowledge he wishes from them. If you're not willing to go so far as to call his authority violence, it is, at absolute least, self-assuredness.
The nameless authority that Roger holds is fear. It is not the authority of an adult as one might assume. There is no respect in his authority. He commands others because he is ruthless. When he kills Piggy, he shows no remorse and smiles sadistically afterward. He enjoys inflicting pain on others and Sam and Eric perceive this and are only submissive because of the pain he can inflict on them.