ya like everyone else said it stress tests your computer so that u can make sure its stable after some overclocking, some people leave it running for 24 hours or something but i think 1 hour is fine to see that its stable
Prime95 is the best stability tester that I've ever used. And, Gaming Freak, one hour isn't really stable. If it is over around 3-4 hours it should be fine. I think I got about 5 hours out of mine.