Net metering is a program that the utility company provides to homeowners who want to go solar. When a home installs solar they are changed into the current net metering program. This allows the home to use solar energy and still be connected to the electrical grid, so if there are ever any issues with the solar system homeowners can still draw power. It also allows for homeowners to sell their excess energy back to the utility company. In some cases this can result in the utility company actually paying the homeowner for the energy they provided at the end of each year.
The Net Metering agreements play a large role in how affordable solar is for each home. The current programs (NEM 2.0) are very friendly to homeowners thinking of going solar. However changes are coming soon to most Net Metering programs in California. Check your utility provider's website for more information on how these changes impact you.