What is meant by the phrase refractive error?

Image focus problem. Refractive error refers to nearsightedness, farsightedness, or astigmatism. If there is an imbalance between the to eyes, it is called anisometropia. These are all problems the eye has in focussing a clear image on to the retina.
Optical problem. Refractive error means an abnormality of the optics of the eye that can be corrected with glasses and/or contact lenses.