The intent of the program is to ensure that Amazon only retains people who really, really want to work at Amazon:
The second program is called Pay to Quit. It was invented by the clever people at Zappos, and the Amazon fulfillment centers have been iterating on it. Pay to Quit is pretty simple. Once a year, we offer to pay our associates to quit. The first year the offer is made, it's for $2,000. Then it goes up one thousand dollars a year until it reaches $5,000. The headline on the offer is "Please Don't Take This Offer." We hope they don't take the offer; we want them to stay. Why do we make this offer? The goal is to encourage folks to take a moment and think about what they really want. In the long-run, an employee staying somewhere they don't want to be isn't healthy for the employee or the company.
You can read the entire letter here.
Disclosure: Jeff Bezos is an investor in Business Insider through his personal investment company Bezos Expeditions.