Bromine comes in granular or tablet form and is an alternative to chlorine. It is more stable than chlorine in warmer water temperatures.
Bromine combines with bacteria in pool water to neutralize it in the same way that chlorine does, however a good portion of the bromine stays active even after combining with the bacteria.
The shock treatment will burn off the bacteria leaving the bromine in the pool water to sanitize the pool again.
Because of this, the level of bromine in the swimming pool water is less than that of the chlorine level. People with sensitive skin will use bromine over chlorine.