A theatre is the place where plays are performed, especially in England, because the normal American spelling of theatre is theater. If you're looking for a night of drama and astonishment, try going to the theatre.
Theatre comes from the Ancient Greek theatron, which means a place for showing, or performing. Now we call the place and the stuff performed the same thing. There are theatres where other things happen than plays. A theatre of war is the area in which a war is taking place. If you're fascinated by war but don't have the nerve to be a soldier, why not make theatre about war?