FR (Flame Resistant, Fire Rated, or Fire Resistant), refers to the ability of a material to resist igniting, and in the case ignition does occur, its ability to keep the flame from spreading from the initial point of ignition.
This is very important when protecting your vehicle because it can make the difference between a small burn hole in the carport versus the entire structure burning if the carport were ever exposed to burning embers from a nearby fire, a burning branch from a lighting strike, or and dry material were to somehow come in contact with a hot part of the vehicle immediately after parking in the carport.
In the case of carports, many of covered-cars.com carports are NFPA 701 compliant. This is a government standard by which all materials must undergo a test where the material is hung vertically and exposed to an open flame. The amount of material burnt must not exceed a certain amount in a specific time.
For more information on the different ratings, and what they mean, you can visit https://foodtents.wordpress.com/2013/11/18/understanding-nfpa-701-cpai-84-ca-fire-marshal-seal-for-food-concession-tent/