It is an insulator which can be polarized by an external electric field
When a dielectric material is placed in an external magnetic field, it gets polarized.
i.e., the positive and negative charges gets aligned in the opposite direction of electric field which causes an electric field to form in the opposite direction.
They modify the electric field in which they are placed in, where is the electric field in the presence of dielectric, is the electric field in the absence of dielectric and is the dielectric constant.
Therefore, the presence of dielectric decrease the electric field