Chlorine in pool and spa water may be present in two forms. It is free chlorine that does the hard work of killing bacteria and oxidizing contaminants. (When you add a chlorine compound like Cal-Hypo or trichlor to your pool, you are actually adding free chlorine.) When the free chlorine combines with these contaminants, such as oils, swimmer waste and other organic compounds, it becomes combined chlorine, or chloramines. In pool and spa water, this form of chlorine has very little sanitizing ability, and no oxidizing ability. You can think of combined chlorine as a spent bullet.
Total chlorine is just the sum of both combined chlorine and free chlorine. In other words,
(total chlorine) = (free chlorine) plus (combined chlorine)
Knowing your total chlorine and free chlorine levels allow you to calculate combined chlorine (combined chlorine = total chlorine minus free chlorine). If the total chlorine level is higher than free chlorine, it is obvious that combined chlorine is present. In that case you need to shock or superchlorinate your pool or spa. To shock the pool, you add a free chlorine compound in an extra large dose. The high dosage of free chlorine will actually oxidize and eliminate the combined chlorine.
A healthy pool should have NO Combined Chlorine! Combined Chlorine is "used up" Chlorine that has "Combined" with swimmer waste and other debris. Combined Chlorine is Chlorine which no longer has any sanitizing capability. Combined Chlorine gives off a strong offensive Chlorine smell even though Total Chlorine levels are relatively low. This usually indicates that the pool needs to be super chlorinated with large quantities of Free Chlorine. If there is no strong Chlorine smell and the water has a crystal clear appearance, you can probably just add enough additional Chlorine to achieve "Breakpoint Chlorination" or "super chlorination". Breakpoint Chlorination requires that you add enough additional Chlorine to equal 10 times the amount of Combined Chlorine. Example: If Combined Chlorine is 0.5 PPM you would add 5 PPM of Free Chlorine. If the water is not otherwise in perfect shape you should Chlorine "Shock" the pool by raising the Free Chlorine level of the pool to 10 PPM.