Dielectrics are polar molecules that increase capacitance

Capacitance & Charge/Discharge — AQA A-Level Physics

A dielectricAn insulating material placed between the plates of a capacitor that increases its capacitance by reducing the electric field strength for a given chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C).. is an insulating material placed between the plates of a capacitor.
Dielectrics are made of polar molecules -- molecules with a positive and negative end.
With no applied field: the polar molecules are randomly oriented.
With an applied field: the molecules align with the field, negative ends towards the positive plate.
The aligned molecules produce their own electric field that opposes the applied field.
This reduces the overall electric field between the plates, which reduces the p.d. for the same chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C)..
Since $C = Q/V$ and V decreases, the capacitanceThe chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C). stored per unit potential difference across a capacitor. Measured in farads (F). increases.

Key Definition

Relative permittivity (dielectric constant) — The ratio of the permittivity of a material to the permittivity of free space: epsilon_r = epsilon / epsilon_0. It is dimensionless.

Permittivity measures how easily a material generates an electric field within it.
All materials have epsilon >= epsilon_0. For air, epsilon is approximately equal to epsilon_0.
The larger the permittivity, the larger the opposing field from the dielectricAn insulating material placed between the plates of a capacitor that increases its capacitance by reducing the electric field strength for a given charge., and the greater the capacitanceThe charge stored per unit potential difference across a capacitor. Measured in farads (F)..