Refractive errors are when the image of what you are looking at is not focussed properly onto the retina. The retina is like a film in a camera. There are four major refractive errors: myopia, hyperopia, presbyopia and astigmatism.
For perfectly clear vision, the image of a viewed object needs to be focussed onto the retina, just as a camera has to be focussed properly in order to take a clear picture. If the image is not focussed exactly on the retina, then the image will be blurred, just like an out-of-focus photograph. In this case, the person is said to have a refractive error.
Myopia is when you have trouble seeing clearly in the distance Myopia FAQs
Hyperopia is when you have trouble focussing up close Hyperopia FAQs
Presbyopia is a common condition that makes reading difficult in middle age. Presbyopia FAQs
When the front of the the eye is oval shaped and means blurred vision at all distances. Astigmatism FAQs