Maybe just another way of saying it.

Free Chlorine does the job of killing bacteria and oxidizing contaminants. When you add chlorine to the water, you are adding Free Chlorine. When the Free Chlorine reacts with contaminants such as oils, bacteria and organics, it becomes combined chlorine. Combined chlorine has little sanitizing ability, and no oxidizing ability. Therefore, if the total chlorine level is higher than free chlorine, that indicates the presence of combined chlorine. In that case you need to shock or superchlorinate your pool.

Jim