A dielectric material (dielectric for short) is an electrical insulator that can be polarized by an applied electric field.
When a dielectric is placed in an electric field, electric charges do not flow through the material as they do in a conductor, but are displaced from their original positions.
Positive charges are displaced in one direction (parallel to the applied electric field) and negative charges are displaced in the opposite direction (antiparallel to the applied field).
These displaced charges give rise to their own electric field that opposes the external field as shown below.
This makes the net field within the dielectric less than the external field.
If a dielectric is composed of weakly bonded molecules, those molecules not only become polarized, but also reorient so that their symmetry axes align to the field.
It is the ability of dielectric materials to reduce the amplitude of an electric field that leads to their most common applications: increasing the capacitance and maximum operating voltage of capacitors.