The eye is a simple design - it works like a camera.
A small hole lets light in the front and the back of the eye has sensors on it which transduce the light to electrical energy to go the brain for interpretation. The first layer of the eye is the cornea, a clear window which has magnifying ability. The eye has a way of protecting this sensitive part - eye lids, which react in a fraction of a second as required. The middle of the eye ball has a gel like substance in it which keeps the lining at the back of the eye stuck on, and keeps the eyeball in its shape without stopping light from getting through. There is a lens which finely focuses the light on to the fovea in the back of the eye, to give clarity.
In the case of myopia or short sightedness (which affects Chinese disproportionately) the light is not able to focus. Long-sightedness is also common, and typically starts to affects adults in the early 40s. These two conditions can happen at any age, regardless of race or gender.
In general these conditions are correctable by external lenses like glasses or contact lenses. Some people get laser surgery to alter the shape of the cornea to allow perfect focus again. People need to weigh up the cost of this (usually in the thousands) and glasses (in the hundreds) and their lifestyle. Usually an optometrist can sort this out and can also screen for other causes like glaucoma pressure build up in eye casing damage to fovea, retinal detachment (the gel not keeping back of eye attached so that it pulls away from blood vessels and nerves) and cataracts (dirty lenses essentially, which are not cleanable). If in doubt, see your doctor to exclude things like strokes from causes of eye problems.