insulator

I suppose the parallel metal plates are charged. If not, there would not be any elecic field in the insulator placed in between.

Electric field is set up between the charged plates. The field induced charges on the dielectric (i.e. the slab of insulator). The side of the dielectric facing the +ve charged plate is induced negatively, i.e. with -ve charges induced. The side of the dielectric facing the -ve charged metal plate is induced positively wth +ve chagres induced.

The induced +ve and -ve charge on the two sides of dielectric would align the atoms, which are in the form of dipoles, in it, thus producing an electric field.