Original: Severe thunderstorms, which can spawn tornadoes and hailstorms, require wind shear to organize the storm in such a way as to maintain the thunderstorm for a longer period of time.
Simplified: Severe thunderstorms, which can tornadoes and hailstorms, require wind shear to organize the storm in such a way as to maintain the thunderstorm for a longer period of time by separating the storms inflow from its rain-cooled outflow.