There are two methods for refinishing hardwood floors
1) Sanding down to the bare wood, then sanding till
the floor is flat and smooth. Apply stain if desired, and applying all finish coats (usually three coats, using low voc wateborne urethane whenever possible).
2) Buffing top of wood floor to physically prepare and create a workable surface for the new finish to bond to. Then, the floor needs to be chemically prepared by removing cleaning agents, grease and oils, as well as other contaminates. After the floor is thoroughly cleaned, 2 coats of finish are applied (2-part commercial finish is recommended).