Although it might seem contradictory, something can be abstract and concrete at the same time. These are relative qualities.

Relative to anarchy, the state is abstract. It depends upon a socially shared model of thinking that gets acted out by individuals. Take away this layer of abstraction and you are left with anarchy, i.e. the precursor of the state.

Prison is itself an abstraction, though its consequences feel very concrete to a prisoner. All of your thoughts and feelings are abstractions, and yet they seem concrete to you.