When a window product has an NFRC rating, it means that the product has been tested by a certified laboratory using standards per the National Fenestration Rating Council (NFRC)

The NFRC rating provides information about the energy efficiency of a window product, including its U-factor, solar heat gain coefficient, and visible transmittance. 

The value of the NFRC rating is that it provides the values for the entire window assembly, as opposed to measuring the center of glass value (which will be vastly different due to the frame effects)

Sometimes, a standards such as ASHRAE 90.1 will define a minimum NFRC rating based on the building type and climate zone.

The U-factor measures how well a window product insulates, the solar heat gain coefficient measures how well a window product blocks heat from the sun, and the visible transmittance measures how much light a window product allows in.