Recommendation for voltage derating means that the actual capacitor shall be used in the application at a lower voltage than the rated voltage. Derating is expressed usually by percentage of rated voltage that shall be subtracted. For example, 20% derating means that the capacitor shall be used at 80% of rated voltage at the specific applications (10V capacitor to be used on 8V maximum).
Electrolytic capacitors are more complicated than electrostatic capacitors in their construction. The function of electrolyte is to provide electric connection to the first electrode with very high surface with fine structure and thus to achieve high capacitance values.