A little hard to explain. Basically, if you are able to generate enough power that you "give" power to the utility company, then they will "pay" you for it by giving you credit.
In this situation, they don't give you money to spend on other things, so it isn't technically a payment. They save the credits for you, and if you ever lose your solar panels, they will use the credits you have accumulated to pay your energy bill for you. So, when the credits run out, you have to start paying again.
Does that make sense?