Electrolytic capacitors use a very thin film of oxide on the positive electrode as the "insulator" between the plates. A small leakage current is required to maintain this oxide layer. If left unpowered for a long period the oxide layer can break down, causing a short circuit. When power is applied in this condition, it may repair itself. If the capacitor IS shorted, the current will be excessive causing the capacitor to heat and generate gas. In some cases, the gas can create enough pressure to cause the aluminum can to explode.
In order to avoid the over heat and possible explosion, re-forming is required. Reforming is simply the process of controled application of voltage and current so the capacitor has a chance to re-oxide.
Lots of great information is available on the web about reforming capacitors and circuits for reforming. Check out some of these links for more details or use your favorite search engine with "capacitor reforming" as the search criteria.