FR stands for Flame Retardant. FR properties can be either inherent in the yarn or applied as a finish. Inherent FR cannot be washed out and generally achieves a high certification for fire retardancy. An after-market or solution finish can also achieve a good FR rating in the short term but may lessen over time depending upon how the fabric is treated.
The responsibility for attaining the appropriate FR certification is the role of the Specifier, as every project and location has a different requirement with varying context.