The ozone layer is a stratospheric region composed of Ozone (Triatomic Oxygen)
CFC´s (Chlorofluorcarbons) are broken down by ultraviolet radioation to release chloride ions wich react with ozone breaking it down into oxygen.
One chlorine atom can destroy 100.000 ozone molecules - smallamounts of CFC can break down large amounts of ozone.
The thinning of the ozone layer by CFCs reduces the absorption of UV light in the stratosphere