A capacitor is a device that stores charges. For a sinusoidal voltage circuit which contains a capacitor, the capacitor will alternately charge and discharge at a rate determined by the frequency of the supply.
When an alternating sinusoidal voltage is applied to the plates of a capacitor in an AC circuit, the capacitor is charged firstly in one direction and then in the opposite direction changing its polarity at the same rate as the AC mains voltage.
