Possible cause of dents in your hardwood floor include high heels, heavy objects dropped on floor and unprotected rolling of heavy appliances such as a refrigerator or freezer. Obviously to prevent this, remove high heels or maintain proper heel-tip protectors. Provide large felt or rubber protectors under heavy furniture legs. Roll heavy casters over plywood protection only. For individual dents where wood fibers are not broken, cover with a dampened cloth and press with an electric iron to draw fibers up. Last resort sand and finish - owner pays.