In a healthy eye light is automatically focussed onto the retina, the layer of tissue at the back of the eye that captures the image of what you’re looking at. Refractive errors occur when there is a difficulty in the focussing power of the eye. Either the light is not focussed on the retina or it’s focussed with difficulty.
A common eye condition, particularly in older people, is when the clear lens inside the eye which focusses light on to the retina becomes cloudy. A cloudy lens is called a cataract. While ageing is the most common cause of cataracts they can also result from birth defects, trauma to the eye, diseases like diabetes and by using medications like steroids.