The mass extinction that most people are familiar with is the Cretaceous-Tertiary extinction, which is when the dinosaurs were wiped out. But there have been others, some of them much worse.
The new theories propose the idea that these extinctions could have been caused by vast clouds of interstellar matter. These clouds might have blocked some of the sunlight reaching the Earth, causing an ice age. Or they could have stripped some of the ozone from the upper atmosphere, increasing the amount of ultraviolet radiation that would reach the surface.