Why would a hole on the ozone cause mutations? And what are mutations?

The ozone layer reduces the amount of ultraviolet (UV) light that reaches the surface of the Earth. A hole in the ozone layer would expose people underneath it to ultraviolet light from the sun. This UV light damages skin cells. One way it damages cells involves DNA. When DNA is exposed to UV light, its sequence of bases can change. Specifically, UV light can cause thymine molecules in DNA to stick together, forming a "kink" in the DNA chain. These changes are called mutations. Some mutations are harmless. Others can cause cancer or other problems. Melanoma is an example of a form of cancer caused by exposure to UV light.