Ozone is a molecule that is three Oxygen atoms.  There is a layer of these molecules surrounding our planet, we call it the ozone layer.  This layer deflects harmful rays from the sun, and the sun is always sending something to kill us.  The hole in the ozone layer is created because of other molecules or elements getting into out atmosphere.  CFC's for example, they are released and float on up to the ozone layer, there the oxygen atoms in a ozone molecule break off and join up with these new chemicals, this depletes our ozone layer.  Hot tub owners are also interested in ozone.