The Fed is buying 600 billion dollars in debt. Will someone who really understands this please explain it in lay terms?

This is obvioulsy a big deal. So, what are the basic mechanics of this? What does it mean to our economy? What are they trying to accomplish? What would failure of this look like? What would its success mean? Please be objective in responding. I'm not trying to spur a political argument, I just want to help inform myself and the reddit community. Thanks.