The rf value is shorthand for the retention value of a substance. It is used in chromatography to determine the components of an unknown sample.

Rf = distance to color/distance to front line

Rf value = distance traveled by a compound/distance traveled by a constant.

Source: http://www.chemguide.co.uk/analysis/chromatography/paper.html

See related link below for a better explanation than mine..

I am not to familiar with Rf values, but I have an idea. Rf values are the ratio between the distance traveled by the spot of the analyte to the distance traveled by the solvent used. This is used in paper chromatography and thin layer chromatography. Rf value will allow you to compare to your unknown substance that you are trying to analyze.

I will put a link the both paper and TL chromatography.

It is the distance travelled by the sample or analyte divided by distance travelled by the solvent front in chromatography.