The ozone layer (O3) is in the stratosphere, about 25 km high, and acts as a filter for ultraviolet rays, preventing them from reaching the earth's surface and damaging the health of living beings.
The ozone layer began to thin out when, starting in the 1970s, CFCs (chlorofluorocarbons) began to be released from aerosols, solvents, refrigerants, and fertilizers. CFCs, by the action of solar radiation, release chlorine and bromine in the stratosphere, which react with ozone and decompose it. This caused that in the 80s, in the Antarctic area (South Pole) it decreased significantly, originating the well-known hole in the ozone layer. It is not really a hole, but a decrease in the thickness of the ozone layer.
In the 90s the emission of CFCs was banned and you can already see how it is increasing its thickness again.