A dielectric slab is a substance which does not allow the flow of charges through it but permits them to exert electrostatic forces on one another.
When a dielectric slab is placed between the plates, the field ${\mathrm{E}}_0$ polarises the dielectric. This induces charge $-{\mathrm{Q}}_{\mathrm{p}}$ on the upper surface and $+{\mathrm{Q}}_{\mathrm{p}}$ on the lower surface of the dielectric. These induced charges set up a field ${\mathrm{E}}_{\mathrm{p}}$ inside the dielectric in the opposite direction of ${\overrightarrow{\mathrm{E}}}_0$ as shown.