In states with Renewable Energy Portfolio Standards (RPS), utility companies are required to get a certain portion of their electricity from renewable energy, creating a market for renewable energy credits. Solar Renewable Energy Credits (SRECs) are tradable certificates that you get for every 1,000 kWh of electricity produced by a solar energy system.
So, for every 1000 kWh of electricity that your system produces, you get one SREC that you can sell to your utility.
You can sell them upfront for a fixed price (many installers give you this option), or you can sell them yourself over time.
More about SRECS:
-Check out SREC Trade: http://www.srectrade.com/background.php