Conceptually, free water clearance should be thought of relative to the production of isoosmotic urine, which would be equal to the osmolarity of the plasma.
A negative value is typical for free water clearance, as the kidney usually produces concentrated urine except in the cases of volume overload by the individual.
How rapidly the kidney clears the blood plasma of a substance (be it water or solute) is the renal clearance, which is related to the rate of urine production.
A free water clearance of zero means the kidney is producing urine isosmotic with respect to the plasma.
Values less than zero imply that the kidney is conserving water (likely under the influence of antidiuretic hormone, ADH), resulting in the production of concentrated urine.