I'm almost positive that it refers to the distance the solvent travels in TLC (thin layer chromatography).
EDIT: A quick wiki read-through somewhat confirms what I said earlier. Rf, or retention factor, is the ratio of the distance the substance travels to the distance the solvent travels. If you remember doing TLC in ochem lab, it's a pretty easy concept to grasp. Say you dab the plate with some random solution and then you let the solvent migrate up the plate, these two components will each travel a unique distance from the "start" line you drew on the plate. The ratio of their distances is Rf.