What is the issue with the ozone layer and why is the ozone layer important?

The ozone layer protects us from the Suns UV-B (ultra violet rays). These rays damage DNA (passing right through skin, etc.) which can cause cataracts, cancer, mutation, reduction in crop yields, and loss of arable land.

These rays heat up the stratosphere, as ozone is also a greenhouse gas. So it serves to make the Earth just a bit warmer than it would otherwise be. But this occurs up high, rather than down on the ground, so it serves to moderate day-night temperature swings too.