Net metering is basically a billing mechanism that credits you, the solar energy system owner for the electricity that you generate and add to the grid (The grid or the electrical grid is an interconnected network for delivering electricity from suppliers to consumers).
For example, during daytime, you generate more electricity more than you consume, the excess energy will be exported to the grid. The exported electricity will serve consumers near your line. During the night or periods that your electricity consumption exceed your solar system's output, your electric meter will roll back to credit the electricity you put into the grid.
Net metering allows you to efficiently manage your energy use resulting to more savings on your electricity bill.