NEM is short for "net energy metering" which refers to the relationship you have with your utility have you've gone solar. An NEM agreement allows you to receive credit from your utility for the excess electricity your system generates.

An example of this is when your home's solar system produces more energy than it uses, it is sent back to the electrical grid, and your utility credits you for this energy. Then, at times when you need more energy than you system provides (for example at night, or during heavy cloud cover) those credits apply to the balance you use.