UV light is higher energy light than what we can see with our eyes. Because it is higher energy, it has more ability to damage things it interacts with (slams in to). UV light gets into our cells and can damage DNA. This can cause problems to occur in the cell, like cancer. Cancer (like skin cancer) is basically cells of the body multiplying non-stop, and without ageing. Which is totally not fair because our good cells have many checks on their growth and reproduction, and eventually age and die.