What is "conditioning of a capacitor"?
Even when not in use, there are chemical reactions going on bettween the anode oxide layer and the electrolyte. This may increase leakage current and reduce withstand voltage over time. 

This effect however is reversable by performing a so called "voltage treatment" (also "conditioning").

Voltage treatment normally involves applying the rated voltage with a resistor in series to the capacitor for a extended period (say 1 hour). This reforms the dielectric.