FR refers to flame-resistant, fire-retardant, flame-retardant, or fire-resistant. These have similar meanings.
Flame resistant fabrics are slow to ignite, burn more slowly, and may self-extinguish when the source of flame or heat is removed. According to the Huntington Beach Fire Dept. (CA): Flame retardant means that the fabric will burn slowly and may self-extinguish when the source of heat or flame is removed. These properties offer a margin of safety which allows a person a little extra time to remove garments, drop and roll to smother flames, or actually put out the fire.
The National Fire Protection Association has put forth a Standard for Flame-Resistant Garments for Protection of Industrial Personnel Against Flash Fire. This is sometimes referred to regarding FR fabrics. It means, among other things, that no more than 4 inches of a fabric can be destroyed by a flame in 2 seconds. Passing a Thermal Stability Test means the fabric must not ignite or melt after 5 minutes in a 500 degree oven. The standard also includes a Mannequin Test which uses a complex of heat sensors. The unfortunate dummy is dressed in the FR fabric to be tested and exposed to a flash fire for a few seconds. Body Burn is not to exceed 50%. Bear in mind, though, that this test may not be carried out exactly the same way by different test groups at this time as it is a newer test. Read about FR rules, regulations and testing standards here.
Working Person's Store carries a wide variety of FR products. Some examples of brands that use FR materials like Kevlar, Nomex, Indura are Carhartt, Black Stallion, Thorogood Boots and Kromer Caps.