A capacitor is a two terminal passive electronic device which is use to store the electrical energy in the form of an electric field.
Inside the capacitor, there are two metal plate which are separated by fixed distance, and a dielectric is placed between the plates.
When the capacitor is connected to the voltage source, the charge starts accumulated in these plates, if positive charge accumulated on the one plate of the capacitor, then negative charge accumulated on the another plate of the capacitor.
The charge storing capacity of the capacitor is measured in capacitance.
These accumulation of two opposite charge on the plate of the capacitor creates an electric field between them.
Now, as the dielectric is inserted between the plates of the capacitor its become polarized, which results in the creation of electric field inside the dielectric, which is opposite to the direction of the electric field between the plates of the capacitor.
Therefore net electric field between the plates of the capacitor is reduces, which cause reduce in the voltage between the plates of the capacitor. This allow capacitor to store more charge by the factor of dielectric constant.