The last step in the series of swimming pool chemistry is to add shock( chlorine) the pool. By adding shock you break down the chloramines and destroy them. Chloramines are created when nitrogen containing organic compounds, such as sun screen, cosmetics, sweat, etc., combine with the chlorine residual in the water. The resulting chloramines  are neutralized chlorine that are no longer providing any sanitizing function and actually cause san unpleasant chlorine odor, cloudy water, eye redness, and feed the bacteria and algae that you are trying to kill.  Once a week use of Shock will reduce the chloramines and keep your pool chemicals working as they should.