A Flashover is a a short from phase to earth.  They are usually very loud (like an explosion) and can cause damage to insulators and hardware fittings.  Flashovers can be a result of allowing contaminants to accumulate on insulator surfaces.  If the contamination is conductive, it will reduce the effective leakage distance of the insulator.  The contaminants on an insulators surface can be fine when it is dry, but if there is a small amount of moisture (but not enough to clean the insulator), the moisture can create a leakage path in the contamination and cause a flashover.