Earth's atmosphere is made up of layers. Starting with the lowest layer and going to the highest, the layers of the atmosphere are as follows: troposphere, stratosphere, mesosphere, thermosphere, and exosphere.
Contained within the stratosphere is a protective layer called the ozone. It protects living things from the more harmful parts of ultraviolet radiation. UV radiation comes in two general types. UV-A and UV-B. UV-A, for the most part, is not harmful to humans. In fact, you need it in order for your body to produce vitamin D. UV-A's harmful effects include sunburn and eye cataracts.
UV-B is a lower wavelength of UV radiation, and it struggles to penetrate the ozone layer in our atmosphere. That's a good thing since UV-B radiation is capable of damaging human cells at the molecular level. Specifically, UV-B can cause shape changes to the DNA molecule. When that happens, transcription enzymes cannot "read" the DNA code properly. That will cause incorrect proteins to produced or proteins to go missing completely. Not to mention a whole host of genetic mutations that can occur.