They are energy performance ratings. Here’s a typical sticker you would see on a new window, with the  five performance rating numbers. Letters in blue have been added so that you can reference each rating in sequence in the explanation that follows. Window ratings are certified by the National Fenestration Rating Council (NFRC). The first two performance ratings, U-Factor and Solar Heat Gain Coefficient, are the most important.

1. The U-Factor is a measure of the insulating ability of the window: how well it prevents heat from entering or escaping through the window. U-Factor ratings generally fall between 0.20 and 1.20. The lower the U-Factor, the better the window is at insulating. To convert a U-Factor to an R-Value (the number used to rate the insulating ability of other building materials), divide 1 by the U-Factor. So, a window with a U-Factor of 0.25 would have an R-Value of 4 (1 divided by 0.25 = 4). A low U-Factor is more important in cold climates, where it is important to keep the heat from escaping .
2. Solar Heat Gain Coefficient (SHGC) measures how well a window blocks the transmission of heat from the sun through the window. SHGC is expressed as a number between 0 and 1. The lower the SHGC, the better the product is at blocking unwanted heat gain, which is particularly important in hotter, Southern climates like Gainesville where the homes are air conditioned part of the year.
3. Visible Transmittance (VT) measures how much light comes through the window, and is expressed as a number between 0 and 1. The higher the VT, the higher the percentage of sunlight that penetrates the window for better daylighting.
4. Air Leakage (AL) measure how much outside air comes into a home through the window. AL rates usually fall between 0.1 and 0.3. The lower the AL, the better the window is at keeping air out. It is an optional rating, and not all manufacturers choose to include it on their labels.
5. Condensation Resistance (CR) measures how well an window resists the formation of condensation. It is expressed as a number between 1 and 100, with higher numbers indicating better resistance to condensation on the window. 

    For more details about the rating system, visit the NFRC website at: http://www.nfrc.org/. Also, see our blog post What are the small slots at the bottom of the outside of my window?

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  To learn more about doors and windows, see these other blog posts:

• What causes sweating (condensation) on the inside of windows in the winter? 

• Is every exterior door of a house required to have a landing outside? 

• What are the small slots at the bottom of the outside of my window? 

• Why does condensation form on the outside of some windows and not others in the morning? 

• Why is the garage door track a white tube? 

• What is the raised metal plate on the floor under the garage door?

• Why do I have to hold down the button to close the garage door? 

• How can I tell if a window or glass door is safety glass? 

• What are the code requirements for safety tempered glass for doors?• 

• Should a front door swing in or out? 

• How many exit doors are required for a house?

• How many exit doors are required for a mobile/manufactured home? 

• Are openable windows required to have window screens? Will windows with no screens pass a home inspection? 

• Can a bedroom door open into the garage?

• What are the building code requirements for a door from the garage to the house?

• Why are window security bars dangerous? 

• What are the common problems you find inspecting windows?

• What is causing a foggy haze on my windows? 

• What does a home inspector check on an electric garage door? 

• What is the tempered label on glass at windows and sliding glass doors called?

• Why is pressure washing double pane windows an expensive mistake? 

• How can I tell if a window or sliding glass door is double or triple pane (insulated) glass?

• Do I need to have two exterior exit doors in my house? 

• When is safety glass required for windows at stairs and stair landings?

   Visit our DOORS AND WINDOWS page for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles.

Sample label above from - National Fenestration Rating Council