In refractive errors the image of the object a person is looking at is not focussed properly onto the retina (the light-sensitive tissue in the back of the eye). For perfectly clear vision, the image needs to be focussed, just as a camera has to be focussed properly in order to take a clear picture.
Refractive errors occur when there is a mismatch between the length of the eye and its optical power. These mismatches usually originate during childhood when the eyes are growing.
The exact causes of refractive errors are still being studied, but it is known that both hereditary and environmental influences can affect their development. Most people have some refractive error, but in most cases the error is small, and does not cause any problems. In fact, the average person is slightly longsighted.
Refractive errors can usually be corrected using spectacles or contact lenses. There are also surgical techniques which can be used to correct refractive errors.