There are two kinds of chlorine: free chlorine and chloramine. Chlorine acts as a sanitizing agent by killing bacteria and oxidizing contaminants. Once chlorine combines with these contaminants, it turns into chloramines, which have no sanitizing ability. Chloramines are a waste product that needs to be removed.
Pool owners should know the total chlorine in their pool. They can calculate the total chlorine value with this formula:
(total chlorine) = (free chlorine) + (combined chlorine)
If your total chlorine level is higher than the free chlorine level, you need to shock your pool. That involves adding an extra-large dose of free chlorine compound to your pool. This helps destroy the combined chlorine. Make sure to use an organic or stabilized chlorine stabilizer, as an inorganic/unstabilized chlorine stabilizer will cause your pH to spike.