A capacitor is an electrical device used to improve the operating characteristics of single-phase induction motors (compressors and fan motors). Run capacitors are rated by microfarads. Most commonly the mircofarads in a capacitor fall below the rating which can affect the operation of the motors they assist. Capacitors 5% below or 10% above the MFD rating are recommended they be replaced.
When your capacitor fails it could render your unit useless unless it is correctly diagnosed. It is important to have a licensed technician assist you in determining if in fact a capacitor is the cause that your air conditioning unit is not working.