The retardation factor, more commonly known as an Rf value, is used to help identify components in a mixture. Tables of Rf values for different pure substances that may be in a mixture are published and can be compared to the Rf values seen in a chromatography experiment (so long as the same conditions and solvent have been used). Each spot in the chromatogram has an associated Rf value.
The Rf value for a spot tells us how far the spot travelled, as a fraction, compared to how far the solvent travelled up the paper in the experiment. Thus Rf values range from 0 to 1.
If two substances produce spots with the same Rf values, they are likely to be the same substance.
We can measure how far the solvent has moved and how far a spot has moved using a ruler. It is important to measure from the baseline, not from the bottom of the chromatography paper! In the experiment shown above, component C has an Rf value of 0.7 approximately.