When dielectric is placed between the capacitor’s plate the charge in the dielectric aligns itself such that the negative charge in the dielectric face the positively charged plate of the capacitor, And the positive charge of dielectric faces the negatively charged plate of the capacitor, which makes dielectric polarized.
Electric field is established inside the dielectric material in the opposite direction of the electric field between the capacitor plates.
Because of the electric field inside the dielectric. the net electric field between the plates of the capacitor is decreases, which cause reduce in the voltage between the capacitor plate. This allow capacitor to store more charge by the factor of dielectric constant. Hence the capacitance of the capacitor is increases by the factor of dielectric constant.