The concept is simple. Homeowners get the panels on their home without upfront fees, and then pay the company for the power which the panel produces at a rate equal or in many cases less than what they would have paid to the electric company. If the panels produce more power than the homeowner uses, they receive a credit.
For the homeowner, the upsides are great. Without and investment, homeowners help the environment by producing clean green energy. They help themselves by lowering their rates for electricity, and if they are frugal with their consumption, or get a lot of sun exposure, they actually get a credit on their electric bill.
For the companies providing the leased equipment, there are advantages as well. The homeowner provides them with a location for installation and a connection to the grid and the local power company. They make money by charging the consumer a price lower than the local power company, but enough to make a profit on the equipment.
The leases are not available to everyone. The companies evaluate homes based on solar exposure to ensure that there will be enough power generated to make the installation cost effective, and the rates from the local utilities company have to be high enough to allow for margin for the lease company.